/IDemoirs of tbe 36oston Societv of Iftatural Tbtatocy. Volume 7. vVi ^ PHYLOGENY OF THE ECHINI, WITH A REVISION OF PALAEOZOIC SPECIES. BY ROBERT TRACY JACKSON. WITH SEVENTY-SIX PLATES. TEXT BOSTON: printed for the society with aid from the gurdon saltonstall fund. January, 1912. ^^ MEMOIRS READ BEFORE THE BOSTON SOCIETY OF NATURAL HISTOKY; BEING A NEW SERIES OF THE BOSTON JOURNAL OF NATURAL HISTORY. VOLUME VII. BOSTON: PUBLISHED BY THE SOCIETY. 1912. PUBLISHING COMMITTEE. REGINALD A. DALY CHARLES S. MINOT WILLIAM G. FARLOW WILLIAM M. WHEELER GLOVER M. ALLEN ^^7 t CONTENTS. Phylogeny of the Echini, with a Revision of Palaeozoic Species. 491 pages, 258 text-figures, 76 plates. By Robert Tracy Jackson. January, 1912. TO THE MEMORY OF THE LATE ALEXANDER AGASSIZ, TO WHOM THE SOCIETY MADE ITS FIRST AWARD OF THE WALKER GRAND PRIZE IN RECOGNITION OF HIS 'REVISION OF THE ECHINI. AND OF ALPHEUS HYATT, LATE CURATOR OF THE SOCIETY, MY BELOVED MASTER AND FRIEND. WHOSE PRINCIPLES OF RESEARCH ARE THE KEYNOTE OF THIS MEMOIR. TABLE OF CONTENTS. Page. PREFACE 9 INTRODUCTION •. 15 TERMINOLOGY 25 PART I. COMPARATIVE MORPHOLOGY OF ECHINI 31 Form of the Test 31 Orientation 32 The Pentamerous System and Variation 35 Structure of the Skeleton and Growth 51 The Ambulacrum of the Corona 53 The Interambulacrum 62 Characters op Basicoronal Plates 69 Imbrication of Coronal Plates 73 Spines 77 The Peristome 79 Ocular and Genital Plates 86 Special Characters of Genital Plates 165 The Periproct 173 The Aristotle's Lantern and Perignathic Girdle 177 PART II. SYSTEMATIC CLASSIFICATION OF ECHINI . 199 PART III. PALAEOZOIC ECHINI 235 Geological Distribution 235 Systematic Descriptions 238 (5) 6 Order BOTHRIOCIDAROIDA Jackson Family Bothriocidaridae Klem Bothriocidaris Eichwald . B. archaica sp. nov. B. pahleni Schmidt . B. globulus Eichwald. Genotype Order CIDAROIDA Duncan Family Cidaridae Gray Miocidaris Doderlein M. keyserlingi (Geinitz) M. camioni sp. nov. Order ECHINOCYSTOIDA nora. nov. Family Palaeodiscidae Gregory . Palaeodiscus Salter P. ferox Salter. Genotype Family Echinocystidae Gregory . Echinocystites Wyville Thomson E. pomum Wyville Thomson. Genotype Order PERISCHOECHINOIDA M'Coy Family Archaeocidaridae M'Coy Eocidaris Desor E. laevispina (Sandberger) . Genot3rpe Archaeocidaris M'Coy A. worthcni Hall A. legrandensis Miller and Gurley A. longispina Newberry . A. glabrispina (Phillips) . A. nerei (Miinster) . A. rossica (Buch) A. agassizi Hall A. illinoisensis Worthen and Miller A. coloradensis nom. nov. A. keokuk Hall A. gracilis Newberry A. aculeata Shumard and Swallow A. shumardana Hall A. edgarensis Worthen and Miller A. newberryi Hambach A. Irudifer White A. norwoodi Hall A. paradoxa (Eichwald) . A. mucronata Meek and Worthen A. dininnii White A . oralis White A. acanthifera Trail tschold A. pizzulana Gortani A. megastyla Shumard and Swallow A. biangulata Shumard and Swallow A. ornata (Eichwald) A. ourayensis Girty CONTENTS 238 A. triplex White .... 238 A. triserialis (M'Coy) 238 A . iriserraia Meek .... 239 A. rankini Young .... 242 .4. prisca (Miinster) .... 243 A . wervekei Tornquist 244 .1. urii (Fleming). Genotype 245 A. halliana (Geinitz) 245 A. muensteriana (Koninek) 245 ^. /orbesiana (Koninek) . 247 A. spinoclavata Worthen and Miller 250 ^4. sp. b. Girty 250 A. clarata (Eichwald) 250 Lepidocidaris Meek and Worthen 250 L. squaniosa Meek and Worthen. Gene 252 type 252 Family Lepidocentridae Loven 252 Koninckocidaris DoUo and Buisserct 253 A'. colUaui Dollo and Buisseret. Geno 254 type 254 A', silurica sp. nov. . . . .■ 255 Lepidocentrus Muller 256 L. rhenanus (Beyrich) 259 L. drydenensis (Vanuxem) 260 L. miilleri Schultze .... 261 L. whitfieldi sp. nov. 261 L. eifelianits Miillcr. Genotype 262 Tornquistellus Berg 263 T. gracilis (Tornquist). Genotype 266 Hyattechinus gen. nov. 266 H. rarispinus (Hall) 267 H. pentagonus sp. nov. 267 H. beecheri sp. nov. Genotype 267 Pholidechinus gen. nov. 268 P. brauni sp. nov. Genotype 268 Family Pai.aeechinidae M'Coy 269 Palaeechinus M 'Coy 269 P. quadriserialis Wright . 269 P. ellipticus M 'Coy. Genotype 270 P. elegatis M 'Coy .... 270 P. (?) minor sp. nov. 271 Maccoya Pomel 271 M. burlinglonensis (Meek and Worthen) 272 M. intermedia (Keeping) . 272 M. phillipsiac (Forbes) 272 M. sphaerica (M 'Coy) 273 M. gigas (M'Coy). Genotype 273 M. gracilis (Meek and Worthen) 274 Lovenechinus gen. nov. 274 L. lacazei (Julian) .... 274 275 275 276 276 276 276 279 280 280 281 281 282 282 282 284 284 285 285 286 288 288 289 290 291 291 291 291 292 295 297 299 299 302 304 305 307 308 310 311 312 314 316 317 321 323 324 326 CONTENTS L. mutalus (Keyes) .... 335 L. nobitia (Meek ami Worlhen) 335 L. missouriensis (Jar'kson). Genotype 337 L. anglicus sp. nov 340 L. seplies sp. nov. .... 348 Oligoporus Meek and Wort hen . 350 U. blairt Miller and Gurley 351 0. coreyi Meek and Worthen 353 0. sulcalus Miller and Gurley . 354 0. halli sp. nov. .... 355 0. danae (Meek antl Wort lien ) . Genotjrpe 350 Melonechinus Meek and Wortlien 359 M. dispar (Fischer von WaUlheini) . 365 M. parvus (Hambach) 365 M. springeri sp. now 366 M. crassun (Hambach) 367 M. indianensis (Miller and Gurle.\) . 309 M. slewarlii (Safford) 371 M. seplenarius (Jackson) . 373 M. obovalus sp. nov. 373 M. liratus sp. nov. .... 374 M. muliiporxis (Norwood and Owen) Genotype 375 M . keepringi sp. nov. 384 M. etheridgii (Keeping) 385 M. vanderbiUi sp. nov. 388 M. giganleus (Jackson) 389 ^amily Lepidesthid.\e Jackscni 393 Lepidechinus Hall .... 394 L. irregularis (Keeping) . 390 L. iowensis sp. nov. L. tessellatus .sp. nov. L. imbrioiins Hall. Genotype Perischodomus M 'Coy P. biserialis M 'Coy. Genotype /■*. itiuKiiscnsis Worthen and Miller Perischocidaris Xeunia>i' P. harteiana (Baily). Genotype Proterocidaris Koniiuk /'. i/ii/iinlcii.-i Koninck. Genotype Lepidesthes Meek and Worthen L. u'orlheni Jackson . L. laens Trautschold L. formosa Miller L. devonicans Whidborne . L. spectabilis (Worthen and Miller) L. coreyi Meek and Worthen. Genotype L. carinaia sp. nov. .... L. colleUi White .... L. extremis sp. nov. .... h. caledonica sp. no\'. Pholidocidaris Meek and Worthen P. intgiilari.s (Meek and Worthen) Genotype P. sp P. tenuis Tomquist . P. ncauria (Whidborne) . /'. gnuiln/i (Julien) . Meekechinus gen. no\-. M . elegans sp. no\'. Genotype 397 397 399 401 401 400 407 408 409 410 412 410 418 418 420 421 423 424 425 430 432 432 434 440 440 441 442 442 443 Incertae Sedis .... Archaeocidaris konincki Desor A. ladina Stache A. scotica Young A. selwyni R. Etheridgc, Jr. A. sin Barrois A. Iraulscholdi Tornfiuist A. sp. Meek and Hayden A. sp. Meek and Hayden A. sp. Stache . A. (?) sp. Worthen and Miller A. sp. R. Etheridge, Jr. . A. sp. Julien A. sp. Julien A. sp. Julien Cidarites tennesseae Troost Echinocrinus anceps T. and T. Austin E. spinosus T. and T. Austin . 440 446 447 447 447 448 448 448 448 448 448 449 449 449 449 449 449 E. slriiilus Eichwald E. sp. Austin Melonitiden Tomquist Oligoporus (?) minutus Beede Palaechinus (?) konigii M 'Coy Palaeechinus regnyi nom. nov. Palaechinus rnbineti Julien Palechinus sp. Tomquist . Palaeocidaris exilis Eichwald Palaeodiscus gothicus Wyville Thoni; Palaeospatangus skiptoni Harte Perischodomus magnus Tomquist Protoechinus T. Austin P. ancips T. Austin. Genotype Rhoechinus sp. Duncan Rhoechinus (?) sp. Tomquist . 440 450 450 450 450 450 451 452 452 453 453 453 453 453 4.54 454 454 8 CONTENTS. Incertae Sedis (conliniml). Xenocidaris Schultzc . A', clavigera Schultzc. Genotype 454 455 X. conifera Schliiter . X. cylindrica Schultze 455 456 Nomina Nuda Archaeocidaris tirolensis Staclie Echiiwcnmis cidariformis (?) T. and T, Austin Echinocrinu/i immum T. and T. Aust Heterocidaris Hall . Hetcrocidnns keokuk Hall Heterocidaris laevispiiia Hall 450 45(3 456 456 456 456 456 Lepidocentrus desnri DoUo and Bnisseret . 457 Lepidocentrus duponli Dollo and IJuisseret . 457 Lepidoccntrus gaudryi Dollo and Buisseret . 457 Meloniles youngi Young .... 457 Palacchinus agassizi Dollo and Buisseret . 457 Palaechinus carpenleri Dollo and Buisseret 458 Palaechinus loveni Dollo and Buisseret . 458 Palaeozok' Fokms Incourkctly Refeiired to Echim Discocystis Gregory 458 D. kaxkaskiensis (Hall) .... 4.58 D. op/alus (Worthen and Miller) . . 4.58 D. sampsoni (Miller) .... 485 Myriastiches Solla.s M. gigas SoUas Palaechinus sp. Mitchell Spatangopsis Torcll . S. costata Torell 458 458 4.58 458 458 458 BIBLIOGRAPHY 459 INDEX 475 ERRATA AND ADDENDA 491 PREFACE. In 1896, I published with Dr. (now Professor) T. A. Jaggar, Jr., a detailed study of Melon- echinus multiporus and at the same time, independently, a general study of Palaeozoic Echini. These papers, which were published together in the Bulletin of the Geological Society of America, were the first attempt to make a comprehensive study of this interesting group of fossil animals. During a year recently spent in Europe, opportunity offered to study most of the European species of Palaeozoic Echini and in many cases the types. In this country similar opportuni- ties have been enjoyed with American species. The great liberality and kindness of all to whom I applied for permission to study material is later acknowledged in detail, but I cannot refrain from saying here that the freedom and liberality with which the choicest and rarest material was placed in my hands and every facility offered for study at home and abroad, was one of the pleasantest experiences of my life. The present memoir is naturally divided into three parts: first, a study of the development and comparative morphology of Echini, based on the young, adult, fossil, and living types; second, a proposed systematic classification of Echini based on the preceding studies; third, a revision of Palaeozoic Echini comprising the systematic description of all known forms. The late Professor Alpheus Hyatt originated most important and novel methods of studj' in Palaeontology. It is both an obligation and a privilege to say that during^ many years, when closely associated witli him as student and assistant, he impressed upon me the important principles of stages in development, senescence, acceleration, and parallelism. During my first sea-urchin studies I had the privilege of his ever ready sympathy and personal help. I have tried to carry out this work on his lines and as he would have done it, so far as in me lay. The Hyatt principles are the pith and center of the work, and I take pleasure in feeling that his views worked out largely on the Mollusca are supported and strengthened by their applica- tion to another group of animals. In revising a group, it is most important to see the types or original specimens on which species have been based. Omitting Incertae Sedis, the types have been studied of 69 of the 119 species of Palaeozoic Echini described in this memoir. Specimens of 96 of the species have been studied. These include almost all of the important forms. The genera and species of Palaeozoic Echini have for the most part been described in the publications of geological surveys as parts of the fauna of a horizon or locality, or as descrip- tions of a limited number of species in the proceedings of learned societies. ■ (9) 10 ROBERT TRACY JACKSON OxV ECHINI. Of those who have published on Palaeozoic Echini, the most important are as follows. M'Coy did pioneer work; in 1844 he published several species of Palaeechinus, the first known, also species of Archaeocidaris, and later, Perischodomus. James Hall described several new species of Archaeocidaris, also Lepidechinus. Meek and Worthen, or Meek independently in a few cases, did most accurate and painstaking work, describing many genera and species with a care and faithfulness that have rarely been equalled by investigators of these ancient and difficult fossils. Sir Wyville Thomson described the remarkable Echinocystites. Bailj- published observations on Palaeechinus and described Perischocidaris harteiana. Schmidt made important studies of Bothriocidaris, describing a new species. Keeping described new species and pubUshed good figures of old species. Duncan made studies of the ambulacrum in species of the Palaeechinidae. C. A. White described a number of new species of .\rchaeo- cidaris and a Lepidesthes. Keyes described a new species of Oligoporus, published a number of new figures, and gave a revision of the American species. Jaekel published a careful study of a species of Bothriocidaris. Jackson and Jaggar published a detailed study of Melonechinus multiporus, and Jackson a general study of Palaeozoic Echini, described a few new species, and made an attempt at a natural classification. Julien described new species and figured others from the Carboniferous of France; his material was evidently not very good as shown by the photographic illustrations. Tornquist made a careful study of German species with most excellent figures; he described a number of new species and genera and gave a classification. Miss Klem published a revision of Palaeozoic Echini with a synopsis of known species. Her bibUography and synonymies are very full and most helpful, but unfortunately there are many errors in the text, so that statements need verification. Fraipont published an interesting series of species from the Carboniferous of Belgium. Besides the above, new species or genera of Palaeozoic Echini or observations on specimens have been published by many authors. Aldrovandus, in 1618, published the first Palaeozoic echinoid that has been discovered in the literature. He figured at this early date a specimen of Bothriocidaris globulus under the caption, "Echinus lapis spoliatus a spinis." It is extraordinary that this especially rare type should have been so early discovered. De Koninck, Miinster, L. Agassiz, Norwood and Owen, Fischer von Waldheim, Forbes, Geinitz, King, Roemer, J. Miiller, and Sandberger published species and genera with more or less full descriptions of the same. Desor published some new forms and gave a revision of the then known Palaeozoic species. Young, Wright, Shumard, Schultze, and Safford published a few types, some of much importance. Lovcn in his "Etudes" gave a revision of the then known species, but apparently without observations on specimens. A. Agassiz in the "Revision" (1872-'74, p. 644-650) gives a general discussion of Palaeozoic Echini and in several other monographs has published observations on the group and some discussions of the observations of others. Stache, Miller, and Schliiter published new species, Zittel in his text-book introduced new PREFACE. 11 generic names and higher divisions of the group. Pomel gave a revision of the genera and published a number of new ones. This part of his work was apparently based wholly on the observations of others, to whom he gives neither credit nor reference. Waagen, and Miller and Gurley published new species. Duncan revised the genera in his revision of the genera of the class. Sollas, Gregory, Whidborne, DoUo and Buisseret, Girty, Bather, and Spencer pubhshed new species or observations on older species. In numerous text-books, figures and more or less full descriptions are of course given, wholly or mainly from the observations of others. The important monographs of A. Agassiz, Loven, and Mortensen have been constantly in hand for comparative studies of recent types. The late lamented Alexander Agassiz stands unique in the history of echinology, for his work on the group extended over a period of nearly fifty years, during which time he published numerous extensive monographs besides many smaller papers. The attempt has been made to give full credit to the above authors for their observations. While I have not always been able to agree with the observations or conclusions of others, it is to be remembered that these fossils have a complex structure and are often very imperfectly preserved, so that they are exceptionally difficult to study. Also, I have had the privilege of a fuller opportunity than probably any one before to study a large range of Palaeozoic species. Those who are most familiar with the group can best appreciate the difficulty of avoiding errors, as I hope may be remembered in my own case. Barring possible oversights, the systematic part of this paper includes every pubhshed species, practically all of which are figured. A figure is given of all recognized species excepting Archaeocidaris rankini, A. prisca, and Konincko- cidaris cotteaui of which no specimen was available and no figure has been published. Also excepting Pholidocidaris gaudryi of which the published photographic figures are too indistinct to be safely copied. The location in museums of types and rare species is given as an aid to investigators. For their kindness and liberality in allowing me freest access to material in collections and for every facility in the use of the same, as well as other kindnesses, I would return grateful thanks to the following curators and private collectors. My friend, Dr. F. A. Bather, of the British Museum, to whom I am under special obligations, most kindly helped me by corre- spondence and the loan of rare specimens, as well as by access to the rich material in the British Museum. He also had the great kindness to borrow, study, and send me drawings and notes on the specimen of Lovenechinus lacazei described from the York Museum. During a visit at the British Museum I enjoyed the privileges of the rich library facilities there available. Dr. F. L. Kitchin and Mr. H. A. Allen, of the Geological Museum in Jermyn Street, London, helped in a similar way, and most kindly had photographs taken of some of their important specimens as here reproduced. Professor Henry Woods, of Cambridge, England, and Pro- fessor John Joly, of Trinity College, Dublin, gave access to their choice material. Dr. R. F, 12 ROBERT TRACY JACKSON ON ECHINI. Scharff, of the Science and Arts Museum, Dublin, gave me access to invaluable material, had casts and photographs of M'Coy's types made, and most generously brought over to this country for studj" one of their most valuable specimens. Dr. Otto Jaekel, recently of Berlin, kindly gave me the opportunity to studj^ his most precious specimen of Bothriocidaris, which I feel is the backbone of this work. Professor A. Rothpletz and Dr. F. Broili gave me every oppor- tunity to study the rich collections of the Munich Museum. At Geneva, Dr. Bedot, Director of the Musee d'Histoire Naturelle, and Dr. Jules Favre, of the same institution, kindly gave me access to their rich collection of Recent and Mesozoic Echini, including the very valuable material of the late M. de Loriol. Professor G. Steinmann and Professor G. Boehm, of Freiburg i. Br., and Professor A. Tornquist, of Strassburg, placed much fine material at my disposal. In this country, for opportunities to study fossil Echini, I am under deep obligations to many curators of museums and to private collectors. Mr. Samuel Henshaw, Curator of the Museum of Comparative Zoology, kindly gave me free access to the rich collections in that institution. From Professor Charles Schuchert, of Yale University Museum, besides other specimens, was borrowed the superb material of HyaUechinus beecheri and rarispinus collected in Warren, Pennsylvania, mostly by the late Professor C. E. Beecher. I am under similar obligations to the late Professor R. P. Whitfield, of the American Museum of Natural History; Dr. C. D. Walcott and Dr. R. S. Bassler, of the United States National Museum; Dr. George H. Girty, of the United States Geological Survey; Professor Stuart Weller, of the University of Chicago; Professor John M. Clarke, of the New York State Museum; Professor William B. Clark, of Johns Hopkins University; Professor J. W. Beede, of Indiana University, who sent me the remarkable new type, Meekechinus elegans and other material ; Dr. A. G. Ruthven and Professor E. C. Case, of the University of Michigan; Professor R. S. Breed, of Alleghanj^ Col- lege, Meadville, Pennsylvania; Professor L. C. Glenn, of Vanderbilt University; Professor A. W. Grabau, of Columbia University; and Mr. C. W. Johnson, Curator of the Boston Society of Natural History. Of private collectors, Mr. Frank Springer generously loaned me his whole collection of about 125 Palaeozoic Echini, including types and other material recently purchased from Pro- fessor G. Hambach, of St. Louis. Mr. Frederick Braun, of Brooklyn, lent very choice material from his private collection; and Mr. E. Kirk, now of the United States Geological Survey, lent several specimens. Mr. J. E. Hj^de, of Columbia University, loaned the remarkable new Lepi- desthes extremis. To all these gentlemen I would render warmest appreciation for their gen- erosity. The actual specimens loaned or used will be referred to as from the institution or collection to which they belong under consideration of the species. For the opportunity to study the great collection of Recent Echini in the Museum of Comparative Zoology, I am indebted to the Curator, Mr. Samuel Henshaw, and to Dr. Hubert Lyman Clark, who is in charge of that department. For similar opportunities in the United PREFACE. 13 States National Museum I am indebted to Mr. Austin H. Clark and Miss Mary J. Rathbun. Professor Verrill kindly gave me the opportunity to study the Recent Echini in Yale University Museum. The authorities at Ward's Natural Science Establishment at Rochester liberally allowed me to study their large trade collection. For opportunities to study Recent material in their charge, I am indebted also to Mr. Roy Miner, of the American Museum of Natural History; Dr. F. A. Lucas, lately of the Brooklyn Institute of Arts and Sciences; Dr. A. S. Pearse, of the University of Michigan; and Professor E. S. Morse, of the Peabody Museum at Salem. Professor Morse also kindly gave me opportunity to study many Japanese Echini that he collected in that country. Several gentlemen have most kindly supplied me with fine series of specimens ; they are : Mr. A. P. Romine, of Bellingham, Washington ; Dr. W. K. Fisher, of Stanford University; Professor W. E. Ritter, of the University of California; Professor E. L. Mark, of Harvard University; Dr. H. C. Chadwick, of the Port Erin Biological Station; Dr. F. D. Lambert, of Tufts College; Dr. Thomas Barbour, of Cambridge, Massachusetts; Mr. Dwight Blaney and Mr. Owen Bryant, of Boston. Dr. Theodor Mortensen kindly loaned me valuable material and helped me by correspondence. In addition to material studied in various institutions, my work has been based on my own collection of some 40,000 Recent and Mesozoic Echini. These I recently gave to the Museum of Comparative Zoology. My Palaeozoic Echini, accumulated during many years and including 100 specimens, are now also in the collection of the same Museum. I would express my warm obligations to my friend, Mr. Samuel Henshaw, Curator of the Museum of Comparative Zoology, for the use of a room and for the use of the great library facilities there afforded. I would express mj^ deep appreciation of the laborious painstaking care and skill with which IVIr. J. Henry Blake made the larger part of the drawings for the plates and text-figures. It is very difficult to get an artist who can draw a sea-urchin correctly, and without Mr. Blake's skill and patience I should have been badly handicapped. Any sea-urchin is difficult to draw accurately, but in the Palaeozoic, one has to contend with the difficulties of imperfections, often the fusion or indistinctness of sutures; the fossils are frequently external or internal molds or siliceous replacements, and these conditions add much to the difficulties of critical stud.y and correct interpretation. The drawings were all made under my eye, and are as faith- ful to the originals as it was possible to make them. No restorations were permitted, except as indicated by dotted lines. Of the text-figures, Mr. Blake drew figures 1 to 239 and 245 to 253, while figure 244 is the work of Mr. J. H. Emerton; figures 240 to 243 were done by Mr. A. H. Searle, and figures 254 to 256 by Mr. William M. Barrows. I would also express obliga- tions to Mr. F. A. Saunderson, of Boston, for the remarkable set of photographs of Echini which are reproduced as heliotypes. Some of the drawings and photograjjhs were made by other artists in Europe or this country, as mentioned in the description of plates. The illustra- 14 ROBERT TRACY JACKSON ON ECHINI. tions in the plates are arranged as nearly as practicable in systematic sequence following the classification here presented. I am under great obligations to my friend, Dr. Hubert Lyman Clark, for his help in veri- fying references, and help with my manuscript and other matters during my absence from home. Miss H. I. Cowdery rendered valuable service as a typewriter by her careful painstaking copy- ing of the manuscript for the printer. The requisite research work and the preparation of this memoir have taken my close attention in available time for some seven years. That it has taken so long has been a source of regret for more than one reason, but careful work cannot be done in a hurry. I adopt the title "Phylogeny of the Echini" for this memior, because the phylogenetic rela- tions expressed by stages in development and by variation are considered throughout the work. As I have shown previously in my "Phylogeny of the Pelecypoda" (1890), and also in studies of Echini (1896), and of Plants (1899), I believe that by following the Hyatt methods of a comparative study of young, adult, fossil, and living forms we can arrive at a series of facts which justify us in accepting them as expressing an approximation to the real genetic relationships of the forms under consideration. The genealogical relations of the Echini, as understood from the results of my studies, are shown in tabular form on p. 209. INTRODUCTION. The principles made use of and the methods of work employed in studying Echini are here treated before starting into the detailed considerations. The one essential principle that has been applied consistently throughout is the principle of stages in development and the constant comparison of these stages with the characters of more or less closely associated types. This principle applied conjointly to fossil and living forms was urged by Professor Louis Agassiz, and was the center of a large part of Professor Hyatt's work. An attempt has been made to bear in mind the characters of the young, adult, and old age, and to compare these with the young or with the adult of simpler or more specialized types, both living and fossil. As far as material permitted, the entirety of the organism has been taken into account. If Echini have taught me anything, they have impressed upon me that classification should be based on the sum of the characters and not on single characters. Herbert Spencer pointed out the distinction that growth is mere increase in size without the addition of differential characters; development is the addition of differential characters. Development proceeds rapidly during the early growth of an organism, but in later life we may get extensive growth without any development. In Palaeozoic Echini, development, that is, the addition of new characters, is often long continued, so that new characters, especially additional columns of interambulacral plates, may be taken on up to or even dorsal to the mid-zone. Then in the dorsal region we come into the zones of localized stages in development, where the young, last added plates are progressively taking on their full characters as they are pushed ventrally by the intercalation of still younger plates on their dorsal border, between them and the apical disc. Dorsally, we also often find senescent stages or characters of regres- sive development, especially marked by the dropping out of columns of interambulacral plates, as in Melonechinus indianensis (Plate 53, fig. 1). In the Palaeozoic Echini, as elsewhere, specimens of a species vary in size, and doubtless this usually means age, but really young specimens are almost unknown. The only very young one seen is Lovenechinus missouriensis (Plate 39, fig. 1; compare with adult, Plate 39, fig. 5). In the absence of the young, youthful stages are gathered from the characters of the plates at the ventral area of the corona, which, as shown in my earlier paper, is an area that preserves the early stages of development in a remarkable degree. Except when destroyed by resorption, this area in a perfect specimen represents the first interambulacral plates built in any corona, and the first ambulacral as well, unless they have passed on to the buccal membrane. Wliile these ventral plates have increased in size since early youth, their presence, number, angles, (15) 16 ROBERT TRACY JACKSON ON ECHINI. and mutual relations show developmental stages which it is felt, from numerous and careful comparisons, can be trusted as stages in development. Stages in development are not limited to the ventral border, but are seen in many types appearing progressively up to or even dorsal to the mid-zone. This is especially seen in the development of the interambulacrum, where new columns come in often very late in the growth of the individual, as shown in numerous cases, for example, Melonechinus multi'porus (Plate 57, fig. 1). Localized stages in development is the principle that throughout the life of the individual, stages may be found in definite parts that are comparable to the condition in the young and to the adults of simpler types of the group. This principle was discovered in and applied to the young plates at the dorsal portion of the interambulacrum in Palaeozoic Echini in my paper, "Studies of Palaeechinoidea, " published in 1896, p. 228. It was applied to Palaeozoic and Recent Echini in my paper (1899) on this principle as a new law in evolution published in the Memoirs of the Boston Societj^ of Natural History, under the title, " Localized Stages in Development in Plants and Animals." In this memoir the principle of localized stages in development is applied to the interambulacrum, but especially to the ambulacrum, where it is found that primitive characters, seen in the nascent plates at the dorsal border of the area, are striking and most valuable for a comparison with the condition in the young and with simpler types in the group. This principle is shown well in the development of young ambulacral plates dorsally in Centrechinus (text-figs. 92, 94, pp. 106, 107) and is seen especially in the family Palaeechinidae (text-fig. 237, p. 231). This principle was found to have a broad applica- tion in various groups of organisms, as ophiurans, crinoids, corals, cephalopods, also plants; and it has been taken up by the late lamented Professors Hyatt and Beecher, by Grabau, Cushman, Ruedemann, Buckman, Hubert Lyman Clark, and Jeffrey, who found it a valuable aid in studying many different groups of animals and plants. Senescence, which was made so much of by Professor Hyatt in his studies of cephalopods, is best shown in Palaeozoic Echini by the dropping out of columns of interambulacral plates at the dorsal or last built portion of the test. In the old individual of Lovenechinus 7nissouriensis (Plate 42, fig. 6) columns 1 and 2 have in part dropped out dorsally, whereas in a younger specimen (Plate 41, fig. 2) they continue directly to the apical disc. Melonechinus giganieus (Plate 59, fig. 14) shows the dropping out of the eleventh or last added column at the dorsal border of the area. Dropping out of columns is shown graphically in the old-age Melonechinus indianensis (Plate 53, fig. 1) where columns 1 and 2 or even more drop out before reaching the apical disc. Hyattechinus beecheri (Plate 26) shows the dropping out of columns 1 and 2 toward the dorsal portion of the areas. Progressive types are those which show in their development to maturity the addition of differential characters only, without the dropping out or disappearing of such characters, unless in senescence. A good case of a purely progressive type is Palaeechinus quadriserialis (Plate 30, INTRODUCTION. 17 fig. 3), which progressively adds characters, but as far as known does not lose any of these additions. It may be said that in general most Palaeozoic Echini arc progressive types. Regressive types are those whicih, after attaining a degree of specialized characters, in later development, and before old age, lose some of these characters, so Ihat what we call the adult, as gathered especially from the characters of the mid-zone, is simpler than its own young. An excellent case of this is Lepidesthes wortheni (Plate 67, fig. S) which in the young has four columns of interambulacral plates, but the fourth column drops out earlj^, and in later life it has three columns only. The sixth column represented by only a few plates in an area seems to mark Lnvenechimis missouriensis (Plate 41, fig. 1) as a regressive type, at least in this char- acter. To go outside of Palaeozoic Echini, Homicidaris has compound ambulacral plates in the lower or youthful half of the test, and above this point has only simple plates, showing a complete reversion to Cidaris, a more primitive type of Echini. Such an extreme case ma}' be compared to Lituites in cephalopods (Zittel, Handbuch der Palaeontologie, vol. 2, text- fig. 519), which, after an early coiled stage, takes on a straight stage directly comparable to the early straight forms of nautiloid cephalopods. Cases could be multiplied and some will be mentioned later, but here it is the object simply to point out examples that illustrate the principles made use of. Acceleration of development, one of Professor Hyatt's most important principles, is abun- dantly shown in Palaeozoic Echini. U.sually columns of interambulacral plates, after the first four columns, are added at considerable intervals, as in Melonechinus muUiporus (Plate 57, fig. 1), l)ut M. giganteus (Plate 59, fig. 14), which is a higher species in the series, adds the fifth, sixth, seventh, and eighth columns earlier than does M. muUiporus, as shown in a detailed study in my earlier paper (189(5, p. 179). Hyattechinus beecheri (Plate 26), a most specialized type, has a very accelerated development, and new columns of interambulacral plates are added so rapidly that the fourth to the tenth are added in succeeding rows, or even two columns may be added in a single row. The same character of acceleration is shown well in Hyattechinus rari- spinus (Plate 23, fig. 1) and H. pentagonus (Plate 25, fig. 1). Acceleration is shown well in the •ambulacrum in Melonechinus (text-fig. 237, p. 231), in which at the ventral border we find four plates in each ambulacrum, while the lower genera of its family have only two plates. Parallelism is an important feature which was much studied by Professor Hyatt, and is of great value in studying Echini. Parallelism is the taking on of a similar character by inde- pendent lines, and is sometimes difficult to distinguish from real genetic connection. A case of parallelism is .seen in the imbrication of plates of Echini. It has been thought that imbri- cation was a sufficiently important character to group together those forms that possessed it, but it is assumed independentlj' in several distinct groups in the Palaeozoic, as well as in the post-Palaeozoic Echinothuriidae. In the family of the Palaeechinidae (Melonechinus, etc.) ambulacral plates on the adradial suture are beveled over the adambulacrals (Plate 45, fig. 5). 18 ROBERT TRACY JACKSON ON ECHINI. A similar beveling is seen in the petaloid area in some clypeastroids, Clypeaster, surely a parallelism only. Accessory pores in genital plates in modern Echini exist as an individual variation occasionally (Arbacia, text-figs. 197, 198, p. 171; Echinus and others). This appar- ently' represents a parallelism rather than a genetic connection with the several pores charac- teristic of these plates in most Palaeozoic genera (Plate 56, fig. 6). Increase in the number of columns of interambulacral plates is taken on independently in many genera. The absence of resorption of the base of the corona, which permits of the retention of the single pri- mordial interambulacral plate in each area, appears as a parallelism in many independent groups, as represented by Hyattechinus (Plate 26), Perischodomus (Plate 64, fig. 2), Pholi- docidaris (Plate 73, fig. 6), Phormosoma, clypeastroids (text-figs. 43, 52, p. 80), and spatan- goids (Plate 3, fig. 15). Resorption is an important process to bear in mind; it occurs continuously in the growth of each individual plate, where there is a constant resorption within the plate concurrently with increase in external dimensions. Resorption of the base of the corona by the encroach- ment of the actinostome is a very important factor, as shown by Loven (1892). In Palaeozoic types, as I showed in my earlier paper (1893), there is either no resorption (Hyattechinus, Plate 26), resorption of apparently one plate only (Palaeechinus, Plate 30, fig. 3), or resorption of several rows of plates (Archaeocidaris, Plate 9, figs. 6-8). Resorption may cut holes directly through the test, as shown by Mr. Agassiz (1872) in the development of the lunules in the recent Mellita sexiesperforatus (Leske). See text-figs. 22-31, p. 70. Variation is an extremely important subject and is graphically shown in sea-urchins. Here we find the striking character of radial variation well developed, as seen especially in the corona and ocular and genital plates. One radius may be more or less accelerated, or may reach a greater or lesser degree of differential development than other radii in the same individual. This is especially marked in the variation of rate of development and number of columns of interambu- lacral plates attained in Palaeozoic Echini, and in the number of ocular plates reaching the peri- proct in Recent Echini. The range of radial variation in one individual may equal the range of variation of different individuals within the limits of the species. In IVIelonechinus typicallj' there are four plates at the ventral border of the ambulacrum, but in one specimen of Melon- echinus muliiporus (Plate 57, fig. 3) it is seen that in area B there arc two plates only. This is the normal character of the ventral border of Oligoporus (Plate 50, fig. 8) and Lovenechinus (Plate 42, fig. 1), the next lower genera of the family. In order to appreciate variation it is of fundamental importance to be familiar with the characters of the associated species and genera of a case in hand, and also the developmental characters of the same. Variation may be fairly classified under five more or less distinct heads : 1. Arrested variation, in which the variant retains characters seen in its own young and INTRODUCTION. 19 typical of the adults of more primitive allies, but characters which are usually eliminated in development. Arrested variants are shown abundantly in the consideration of ocular plates, when fewer plates may become insert than is characteristic of the species (Tripneustes, text- figs. 123-125, p. 124). 2. Progressive variation, in which the variant has characters not typical of the species, but which are further evolved on the direct line of differential development, and are seen typi- cally in more evolved nearly allied species or genera. Such progressive variants are also shown abundantly in the consideration of ocular plates, where more plates become insert than is typical in the species (Centrechinus, text-figs. 93-95, p. 107 and 176, p. 153). 3. Regressive variation, in which the variant takes on characters of the adult of some simple and more primitive type of the group. Such characters are not necessarily a repetition of youth- ful characters but may go back to a remote ancestry. An arrested variant in a sense is one form of regressive variation, but a regressive variant includes much more than arrested variation. To distinguish them, an arrested variant is one that has developed to a certain point as usual, and then failed to take on the later added characters typical of the species, so that, although an adult, it has immature characters. A regressive variant is one that has attained full char- acters and then in later life has reverted to youthful or primitive characters as an individual variation, or it is a variant that from youth has primitive characters not normally seen in the development of the species. A modern horse with extra digits as in Tertiary times could be considered a regressive variant, but could not be considered an arrested variant. In Echini, cases of regressive variation are shown in the simple ambulacrum in Melonechinus (Plate 57, fig. 3), and in the single column of interambulacral plates in Arbacia (Plate 4, fig. 11) and Trip- neustes (Plate 6, fig. 4). 4. Parallel variation is where a character is taken on exceptionally which may be com- pared with characters normally occurring in some type of the group not closely connected, so that it cannot be genetically compared. A case apparently is the extra genital pores fre- quently developed in Recent Echini (Echinus, text-fig. 115, p. 117). 5. Aberrant variation is where a character is taken on which is quite abnormal, not to be correlated with the typical condition in associated forms. Cases of aberrant variation are sea-urchins which have four or six areas developed (Plate 6, figs. 1-4). All the evidence goes to show that aberrant variation is rare, and most variants can be considered as arrested, progressive, regressive, or parallel variants, and as such can be correlated with species more or less nearly allied which typically possess the character which is a variant of the case in hand. This holds true as far as known in both animals and plants, as seen in the present paper, and as I showed in detailed studies of plants (1899). Next to stages in development, variation is the most suggestive line of study in attempt- ing to work out the genetic relations of plants and animals. When a specimen of one species 20 ROBERT TRACY JACKSON ON ECHINI. has characters approaching those of another, hybriditj' is often assumed as a cause. I have found numerous cases of variation to the character of another species where hybridity cannot be considered on account of geographical separation, and it is feU that we should be extremely cautious in assuming hybridity when the same results might be attained by variation. From my studies on Echini, all the evidence goes to show that variation is in perfectly definite lines, mostly arrested or progressive in character. When variatTon is aberrant, it still follows defi- nite lines of aberration, and sporadic variation is very rare. This opinion is based largeh' on a detailed study of genital and ocular plates which were carefully examined in over 50,000 specimens of Echini (text-fig. 176, p. 153; p. 164). To the late Professor Sven Loven all students of the Echini owe a great debt for the keen ej^esight and insight with which he studied these fascinating animals. His method was largely a critical detailed study of plate structure and relations, and this method I have attempted to follow in the study of Palaeozoic forms. Frequently sutures are so difficult to see and speci- mens are so contorted or badly preserved in these ancient forms, that I can hardly hope to have escaped mistakes; but this method has been followed out consistently and with every effort faithfullj' to describe and figure the specimens. Loven worked with Recent or post-Palaeozoic Echini, in which the axes are known from the presence of the madreporite as well as in many t^ypes by the bilateral symmetry. With •known axes he devised a nomenclature of areas which is of very great value and convenience in brevity and clearness of description. He numbered the ambulacral areas from I to V, Roman, and the interambulacra from 1 to 5, Arabic. The enumeration passed from left to right, re- volving like the hands of a watch, with the specimen viewed from below and the odd anterior ambulacrum being III. This nomenclature is considered more fully under Morphology. It is readily seen in its application to Goniocidaris (Plate 2, figs. 1-3). In dealing with Palaeo- zoic types, there was usually no means of ascertaining the axes, for the madreporite was seen in only four species. Since, in the absence of known axes, Loven's system could not be used, a system of nomenclature like that given in mj^ earlier paper is followed. Taking any inter- ambulacrum as A, the several areas are lettered from A to J inclusive, revolving like the hands of a watch, with the specimen viewed from the dorsal side. When a specimen is viewed from below, the letters naturally revolve in the reverse order. This is shown in its application to Lovenechinus (Plate 39, figs. 4, 5). It may be well to call attention to the application of this nomenclature of areas to speci- mens viewed from different points so that it may be clearly in mind. External views, or in- ternal molds, viewed dorsally: the axes revolve clockwise, from left to right (Maccoya, Plate 34, fig. 4; Lovenechinus, Plate 39, fig. 5). External views, or internal molds, viewed ventrally: the axes revolve anticlockwise, or from right to left (Melonechinus, Plate 55, fig. 1; Lovene- chinus, Plate 39, fig. 4). In molds of the exterior, if dorsal: the axes revolve anticlockwise, from INTRODUCTION. 21 right to left, as Hyatkchinus rarispinus (Plate 23, fig. 3) and H. pentagonus (Plate 25, fig. 3). In molds of the exterior, if ventral : the axes revolve clockwise, from left to right, as in Hrjatt- echinus rarispinus (Plate 23, fig. 1) and H. pentagonus (Plate 25, fig. 1). Orientation, not only for the sake of the axes, but also for the correct position and introduction of columns of plates and direction of imbrication, must be borne in mind in a reversed specimen; for naturally confusion results unless this is taken into consideration. It was a strong temptation to reverse the drawings, so that external molds seen from the interior might appear as if seen from the exterior. It would have simplified matters as regards orientation, but then the drawings would not have corresponded with photographic figures of the same specimen ; also the com- plication is so great that I feared to make errors by reversing drawings. The terms molds and casts are often used indifferently, but it is necessary to distinguii^h them. A mold, either internal or external, is an impression of a fossil and represents the object in reverse. A cast, either natural or artificial, is a mechanical filling of the mold and represents therefore a replica of the original. A pseudomorph differs from a cast simply in that it is a molecular chemical replacement of the original, as in silicification, instead of a mechanical filling of a mold, as in the case of mud, sand, or plaster. Most fossils, when they are not the original skeleton, are either pseudomorphs or molds; natural casts, strictly speaking, are com- paratively infrequent. In the figure of H yattechinus rarispinus (Plate 23, fig. 1) the test is wanting, and the view represents an external sandstone mold of the ventral side, together with an internal mold of the dorsal side, seen from above. The mold of the ventral side, being external, shows the tubercles and peripodia of the ambulacral pores, but the mold of the dorsal side, being an impression of the interior, has no tubercles, and the ambulacral pores are represented by vertical plugs. This shows that the point of view must be constantly borne in mind and orientation carefully considered. As an aid in orientation, a rubber ball was marked with ambulacral and inter- ambulacral areas, then cut down on these lines, and marked within. Also dorsal and ventral plaster molds of a Cidaris were made and lettered as models, for it was found difficult to keep orientation clearly in mind in reversed views, as are external molds. An interesting matter is the condition of preservation of Palaeozoic or other fossil Echini. I have never seen a fossil sea-urchin in which the test was preserved in its original condition, but rather some chemical change seems always to have taken place even in late Tertiary speci- mens. In other groups of animals, as fossil molluscs, corals, etc., the original skeleton is often preserved without any change except the leaching out of organic matter. The sea-urchin skeleton is composed of such loose network-like structure, that it seems to be peculiarly open to chemical readjustment and change. When the skeleton is calcified, the original microscopic ' structure is, at least usually, quite destroyed. The skeleton is often replaced by silica, when we may get very beautiful pseudomorphs preserving the finest details; an example of this is Melonechinus giganteus (Plate 60, fig. 3). 22 ROBERT TRACY JACKSON ON ECHINI. One of the most interesting and instructive methods of preservation is when the surround- ing matrix, both internal and external, is completely silicified and the original skeleton is entirely gone, but is represented in finest details by molds in silica. Such an example is shown in Loven- echinus missouriensis (Plate 39, figs. 4, 5). Here it is seen that everything that was not, is represented bj^ the silica. This is rather a paradoxical statement, but fairly represents the facts. The plates are not preserved, but the sutures between the plates are represented by vertical siliceous ridges; the pores, both ambulacral and genital, are represented by vertical tubes, which can be actually seen (Plate 43, fig. 5) passing from the proximal to the distal side through the space which the plates originally occupied, but now simply a hollow. When the external as well as the internal matrix is in hand (Plate 44, figs. 3, 4), we find on one side, the internal matrix, a mold of the proximal side of the plates; on the other or external matrix we get the distal mold of the plates showing spine tubercles and other external features. Sili- ceous molds of the interior or exterior may be found in which there are no ridges representing spaces between the plates, but the difference of conditions between these two types is not understood. Molds, either internal or external, in fine sand or calcareous clay, are often ex- tremely clear and may be accompanied by more or less of the shell of the test, or this may be wanting. In a specimen of Palaeechinus quadriserialis in the British Museum, the test is crystalline calcite, but only part of the plates are preserved; most of the rest are, however, represented by impressions of their proximal faces on the matrix, as shown in Plate 29, fig. 1, and Plate 30, figs. 1, 3. Sandstone molds may be so fine in details of preservation as to show the outline of plates, tubercles, pores, and even peripodia. Internal sandstone molds show only impressions of the proximal sides of plates with pores in relief as slight plugs. An excellent example is Hyatt- echinus beecheri, a unique specimen (Plate 24, figs. 5-8), which shows the details of outline of the proximal faces of the plates so clearly that I am enabled in Plate 2G to represent the sea- urchin spread out, in which every plate was measured and the angles counted. External sandstone molds are even better, for, on the exterior, details of structure are more strongly marked; tubercles and peripodia exist as depressions (the impression being in reverse), and ambulacral pores as slight elevated plugs. Examples are shown in Hyattechinus rarispinus (Plate 23, fig. 1) and H. pentagonus (Plate 25, figs. 1-4). A curiously complex case is the Hyattechinus rarispinus previously mentioned (Plate 23, fig. 1), in which is seen an external mold of the ventral side, showing tubercles and peripodia, and in the same view an internal mold of the dorsal side. The counterpart of this remarkable specimen (Plate 22, figs. 1, 2) shows just the reverse condition; that is, an internal mold of the ventral side and an external mold of the dorsal side. The test, which was extremely thin, is mostly wanting, but is existent on the margins. The dorsal and ventral sides are brought almost in contact, yet the sandstone did not close up, but retained as a thin interspace the vertical space originally occupied by INTRODUCTION. 23 the thickness of the test with some sand which filled the interior of the test, thus making an internal mold possible. Loven devised the incomparable method of representing the structure of a sea-urchin by a figure drawn from the ventral view and with the several ambulacral and interambulacral areas spread out flat in one plane in a star-like fashion. Of course in the Palaeozoic it is only rarely that one finds a fossil sufficiently perfect to admit of such treatment; but I give figures drawn by this method of Bothriocidaris (Plate 1), Hyattechinus (Plate 26), Archaeocidaris (Plate 10, fig. 10), Palaeechinus (Plate 30, fig. 3), Lovenechinus missouriensis (Plates 40, 41), Lovenechinus sepiies (Plate 45), and Melonechinus (Plate 57). These genera are fairly repre- sentative of the several groups of Palaeozoic Echini, and the figures show the characters of the plates and the method of introduction of columns better than by any other method. Wliile perfect specimens are desirable and most highly prized, yet a great deal can be made out of fragments when studied carefully. A left or right half of an ambulacrum repre- sents the character of the whole. A piece out of the mid-zone, that is, halfway between the mouth and middle of the periproct, represents the area where full specific characters are devel- oped in both the ambulacrum and interambulacrum, and many species are represented by a figure taken from the mid-zone (Plate 47). Such a figure does not show the development as seen ventrally, or the apical disc, but these features are often wanting, or shown in related species, so that a figure of an ambulacrum and interambulacnmi from the mid-zone gives for these areas the essential specific criteria. The skeletons of Recent sea-urchins are often very fragile, and even if not so, the peristomal and periproctal plates are easily injured and lost. It has been found a great help to diji the specimens in, or brush them over with, a dilute solution of shellac in alcohol, or gelatine dissolved in water. Specimens so treated are firm and solid and will stand even rough treat- ment. A specimen with the spines all in place may be soaked in this manner by dipping, and on account of the porosity of the skeleton, there will be no external evidence of the treat- ment; the spines will be firmly fixed in place, and the specimen is in much safer condition for storage or study. Where sutures are difficult to see, as in minute recent material, or ambu- lacral details, it has been found a great help to wet them with benzole. When drying out, sutures stand out clearly that are nearly or quite unrecognizable in the dry state. In prepar- ing fresh material of Echini, I have found that a satisfactory method is to soak the specimens over night in a considerable volume of fresh water to remove the salt, then immerse for a few minutes in boiling water containing a liberal amount of corrosive sublimate in solution. This treatment coagulates the albumen, poisons the specimens, and does not affect the color. They will dry out without odor, and in excellent condition as museum specimens. The method has the advantage of cheapness and quickness. When specimens are to be preserved in alcohol, the treatment with fresh water is also desirable, as it kills the animal nicely relaxed, besides removing much of the salt. 24 ROBERT TRACY JACKSON ON ECHINI. The theoretical views expressed in this paper are supported by a sufficient basis in facts, so that they appear reasonable to me; Init I have endeavored to keep the descriptions of struc- tural detail and the conclusions drawn from them distinct, so that the facts will stand for them- selves, whether the deductions are accepted or not. TERMINOLOGY. Some new terms are made use of and old terms are not used uniformly by all investigators, so that the principal terms employed are given here in brief. Test: the whole skeleton including the spines; or without spines, denuded test. Fre- quently test alone is applied to the skeleton in which spines are more or less wanting, the com- mon condition in fossils. Corona: the portion of the test, including the ambulacral and interambulacral plates, extending from the periphery of the peristome to the ventral margin of the apical disc. In describing an ambulacrum or interambulacrum, the terms right or left are frequently used. They mean the right or left side as seen facing the given area, viewed from the exterior and with the dorsal portion uppermost. Ambitus: the zone of greatest circumference as viewed from above. The ambitus may coincide with the mid-zone, Maccoya sphaerica (Plate 32, fig. 5), or it may lie ventral to the mid- zone, Hyattechinus beecheri (Plate 24, fig. 7), or Clypeaster; or rarely dorsal to the median zone, Lepidesthcs coreyi (Plate 66, figs. 8-10). A sea-urchin is divisible into a series of zones which theoretically are as many as there are horizontal rows of plates, and each zone, at least in earlier life, differs or may differ in char- acter from the preceding or succeeding zone. This is shown graphically in Hyattechinus beecheri (Plate 26) ; here in the first zone of interambulacral plates there is one plate in each area, in the second zone two plates, in the third three plates, and so on up to the introduction of the full number of columns of plates, above which we reach zones of senescence, where plates begin to drop out. In practice, radial variation interferes somewhat with the perfect zonal expression, as all the areas do not develop with exactly the same degree of rapidity. For example, in Lovenechinus missouricnsis (Plate 41, tig. 1), the fifth column originates in the fifth row above the initial plate of the fourth column in area A, whereas in (he other areas it origi- nates in the third or fourth row. Similar variations in rate of development are frequent in later added columns, as shown (Jackson and Jaggar, 1896) in detailed stiulies of Melonechinus multiporus, and as shown here in many figures. A column of plates may be quite wanting in one area, though developed in other areas of the same specimen. In Hijattechimis rarispinus (Plate 23, fig. 3) there are only eleven columns of plates in areas A and G, while there are twelve in area I and thirteen in C and E. In Lovenechinus missouriensis (Plate 41, fig. 1) the sixth column is represented by one or two plates in areas E, G, and I, i)ut is wanting in A and C. A single zone is of course greatly modified in certain areas in bilateral types, as in ambu- (25) 26 ROBERT TRACY JACKSON ON ECHINI. lacrum III in many spatangoids. Such extreme zonal modification does not occur in normal regular Echini. The term zones has been sometimes applied as "ambulacral zones," but this seems an unfortunate use of the word, for such areas do not surround the test, as a zone should, and moreover are not parallel to the equator, which also the term commonly implies. Three main zones may be recognized in any sea-urchin, which I would designate as the basicoronal zone, mid-zone, and placogenous zone. In the basicoronal zone occur the basicoronal plates, the first row of ambulacral and interambulacral plates of the corona bordering on the peristome or buccal membrane. Loven called these plates peristomal plates, but it seems an undesirable use of the word, which is usually applied to the buccal membrane and plates on the same. I therefore introduce the term basicoronal as meaning exactly what is intended, and not to be confused with terms applied to other parts. The basicoronal plates are of special interest as being the oldest or first formed plates occurring in any given corona, and also they are the plates giving support directly or through the perignathic girdle to important muscles of the lantern (text-figs. 22-31, p. 70; and 221-230, p. 193). The median zone, or mid-zone for brevity, is that horizontal zone measured halfway between the poles; it may coincide with the ambitus or lie dorsal to it, or rarely ventral to it. The mid- zone is important in Palaeozoic and most other Echini, as the area in which in the ambulacra and interambulacra the differential specific characters are typically most fully developed. Ventral to it the full features may not have been attained, and dorsal to it we soon get into the area of localized development, where the young plates have not attained the full characters of the species. In clypeastroids and spatangoids that have petaloid areas, these lie dorsal to the mid-zone, so that in such types special consideration of the dorsal area is necessary, but it does not invalidate the mid-zone as an area of importance in the consideration of structure. The placogenous zone (■"'^af = plate, + yeved = birth) is that portion of the corona next to the apical disc in which the newly added coronal plates occur both in the ambulacrum and the interambulacrum. This zone is of importance as the plates in it quite generally present youthful characters, which, as localized stages in development, can be compared with the characters seen in the young, or in the adults of simpler tj'pes in the group (text-fig. 237, p. 231). Roiv, as applied to ambulacral and interambulacral plates, refers to plates lying in one horizontal plane or zone, in contradistinction to columns. Column, as applied to ambulacral and interambulacral plates, refers to plates lying super- posed in a vertical series. The column is of course made up of nothing other than plates of a succession of rows, but is a convenient distinction to maintain in Palaeozoic types, where we find in various forms from two to twenty columns of ambulacral or one to fourteen columns of interambulacral plates. The column, especially in the interambulacrum, usually consists of a very definite succession of plates forced into a continuous series by the mechanical impact TERMINOLOGY. 27 of adjacent plates in the several succeeding rows. In the figures, the columns of interambula- cral plates are numbered from one upward (Plate 57, fig. 1; text-fig. 246). Adradial suture, a term introduced by Bather (1909a), is the suture between the interam- bulacrum and the ambulacrum. As a corollary, in Palaeozoic Echini, when there are more than two columns of interambulacral plates, those lying next the ambulacra are called the adradial plates. The term adambulacral plates or columns has been used, but the term adradial has the advantage of brevity. Median suture is the suture in the median line of the ambulacrum. When there are only two vertical columns of plates, it is perfectly obvious, but when more, it is distinguishable by the size of the two medial columns, as in Melonechinus (Plate 56, fig. 4) ; or if the plates do not differ in size, it is distinguished by the fact that the pores in the plates are set on the side toward the next adjacent interambulacrum and therefore are set on opposite sides of the plates on either side of the median sulure, Lepidesthes wortheni (Plate 67, fig. 8). The term median suture in Recent Echini is ordinarily applied also to the interambulacrum, and in Recent foims it is applicable, as there are two columns of plates; but, taking sea-urchins as a whole, it is not applicable to the interambulacrum; for it is only a coincidence of ordinal or specific character, if there is an even number of columns, and when there is, as in Palaeechinus quadri- seriaHs (Plate 30, fig. 3), the suture in the median line is not distinguishable from any other interambulacral suture except the adradial. In considering ambulacral plates, a certain degree of nomenclature is convenient. Pri- mary plates extend from the middle of the ambulacrum to the interambulacral suture (Palae- echinus, text-fig. 9). Demi-plates extend from the interambulacrum inward, but do not reach the middle of the ambulacrum. Occluded plates extend outward from the middle of the am- bulacrum, but do not reach the interambulacrum. Demi- and occluded plates are typically shown in Lovenechinus (text-fig. 11). Isolated plates do not reach either the interambulacral suture or the middle of the ambulacrum; such plates are extensively developed in Melonechinus and Lepidesthes (text-figs. 13, 14, p. 54). This nomenclature was based on the elements of compound plates in the Centrechinoida,' but it seems that it may equally be applied to the complex arrangement of simple plates in Palaeozoic Echini. ' The name Diadema, given by Schynvoet in 1711, cannot stand for a genus of echinoderms. The first post-Linnaean use of the generic name Diadema is in the "Museum Colonnianum," a dealer's catalogue published in London, in 1797; of this work, the author's name does not appear on the title page, but in the British Museum copy it is stated on two author- ities that the author was G. Humphreys. There are 12 species listed under Diadema, but of these only one is recognizable, as it is stated to be the same as Echinus esculentus Linne. On this evidence, if this work should be accepted, which is very doubtful, the genus Diadema would become a synonym of Echinus, as esculentus is the type of that genus. The next post- Linnaean use of the name Diadema was by Schumacher, 1817, who gave this as a generic name for a cirriped, basing it on Lepas diadvma Linn6, which Schumacher (1817, p. 90-91) changed to Diadema vulgaris. This name is a synonym of Coronula diadema Oken (Lehrb. Naturg., 1815, part I, section 1, p. 3G0). Ranzani, in 1820, also used the generic name Diadema for a cirriped, adopting, as did Schumacher, Lepas diadi-imi Linn6, as the type and giving it the new name Diadema candidum. 28 ROBERT TRACY JACKSON ON ECHINI. A simple plate is one which has held its original character of one undivided and independent piece. All plates in Palaeozoic Echini are of this character. , A compound plate is one made up by the coalescence of adjacent plates, seen in the ambula- crum in certain groups, in which area alone is it known to exist. It is a character of the ambu- lacral plates in the Centrechinoida and some Holectypina, but is unknown in the Palaeozoic. The statement that compound ambulacral plates occur in Palaeozoic types has been made (Duncan, 1889a, p. 13), but it is an error; no such plates occur in any known type. Occasion- ally in regular Echini two genital plates, or a genital and an ocular, may fuse when the resultant may in a sense be considered a compound plate (text-figs. 186, 195, 196, pp. 168, 169). Divided plates are those derived by the splitting on lines of solution of a previously continuous plate. This has been shown by Mr. Agassiz (1904) in the genital plates of Phor- mosoma. According to him (1904, p. 96), splitting occurs in interambulacral and ambulacral plates in Phormosoma. Mr. Agassiz (1874, p. 642) attributed the formation of compound ambulacral plates in the Centrechinoida, etc., to the splitting up of original plates. He also says (1883, p. 17) that anal plates in Palaeozoic Echini are formed by splitting. I have had this possible origin of plates in mind, but in my studies have seen no evidence of the origin of plates by splitting in Palaeozoic or other Echini excepting genital plates in echinothuriids, and rarely the same or ocular plates as variants (text-figs. 190-194, p. 169). When the antero-posterior axis is known, Loven's nomenclature of areas is adopted, num- bering the ambulacra I-V and the interambulacra 1-5. In Palaeozoic species the axes are rarely known on account of the usual absence of madreporic pores. The axes are known from the presence of madreporic pores in Lovenechinus lacazei (text-fig. 240), Lepidesthes formosa (Plate 68, fig. 5), L. colletti (Plate 71, fig. 1), and Meekechinus elegans (Plate 76, fig. 1). Axes are assumed from bilateral symmetry in Hyattechinus beecheri (Plate 24, figs. 5, 8), from the orientation of primordial ambulacral plates, Bothriocidaris archaica (Plate 1, fig. 1), and from the apparently eccentric anal area, Echinocystites (Plate 20, fig. 1). In other Palaeozoic types the axes are oriented arbitrarily by letters A to J, as discussed in the Introduction. Dr. Bather called my attention to a paper by M'Clelland (1840) in which he recognized that the name Diadema is not available for a sea-urchin, but he did not propose a new name. To quote his words, p. 170, "We luckily get rid of the genus Diadema [for Echini], from the term having been previously applied to a genus of Girrhopoda by Ranzani." High authorities reject Muerchen's (1774) names, therefore his Anademais unavailable. The name Calmarius annellaia is given by A. Agassiz (1872, p. 104) as a manuscript name of Gray's, and is treated by Agassiz as a synonym of Diadema selosum. As there is a closely related species, Echinolhrix calamaris {Diadema calamaris Gray), it seems probable that Gray's generic name was Calamarius, taken from that species. Calmarius is evidently the same name as Calaniarius with the letter a omitted by error; on this basis it is preoccupied by Calamaria Boie, 1827 (reptiles). As a new name is thus necessary for this genus, I propose Centrechinus (Ke'vrpov, a sting, and e'xivos, sea-urchin) with C. selosus (Leske), from Key West, Florida, as the type. The Florida and \\est hulian Diademas are considered as Diadema anlillarum Philippi by \. Agassiz and Clark (190S), but here antillarum is considered a synonym of selosum. As the name Diadema cannot stand for an cchinoderm, being preoccupied for a crustacean, the family and ordinal names derived from that genus cannot by the rules of nomenclature be retained. They can be replaced by names from the typical genus and may be called Centrechinidae and Centrechinoida. TERMINOLOGY. " 29 The term peristome is not used in Lov6n's sense, l)ut in the more usual sense of the buccal membrane with its plates extending from the mouth to the basicoronal plates. The peristome may be more or less plated with ambulacral plates only, with ambulacral and non-ambulacral plates, with non-ambulacral plates only, or may he a naked membrane (text-figs. 40-54, p. 80). Primordial ambulacral plates: these are the first formed ambulacral plates, ten in number, primarily situated around the mouth in the buccal membrane (Plate 3, fig. 11), or secondarily as basicoronal plates of the corona, as is usual in clypeastroids and spatangoids (Plate 3, fig. 15). Primordial interambulacral plates: these are the first formed interambulacral plates, five in number, one in each area, which may be retained at the base of the corona in the adult, or may disappear by resorption (text-figs. 22-31, p. 70). Ocular and genital plates are used in preference to radial and basal, as not implying an homology with those plates in crinoids. The sum of these plates may conveniently be termed the oculo-genital ring. Gills are external and interradial in position, situated on the border of the peristome, where they may well be called peristomal gills (text-figs. 55, 56, p. 83) ; or they are internal and radial in position, as Stewart's organs, Cidaris; or they may be external as modified tube- feet, when they may be called ambulacral gills to distinguish them from the interradial peristomal gills. The terminology applied to spines is necessarily rather arbitrary, as such great difference in size exists, but it is convenient to have some basis for description. Primary spines are the large spines occurring in such types as Cidaris or Arbacia. Secondary spines are best represented by the smaller spines clasping about the base of the large ones in Cidaris. Miliaries are minute spines seen between the smaller ones in Cidaris, or all the spines of Echinarachnius could be classed under this head. These are spines so small that it requires a microscope to see their structure. In the Palaeozoic we find primary spines and secondary spines in Archaeo- cidaris (Plate 9, fig. 13) and Lepidocidaris (Plate 17, figs. 10-14). In some types secondary spines only are known, as in Melonechinus (Plate 52, figs. 10, 11) and allies. Mihary spines are so far known in Archaeocidaris only (Plate 11, fig. 4). The points of attachment of the spines are the tubercles which may be primary, secondary', etc., according to their size. The consideration of nomenclature of the parts of spines, plates, etc., of Echini is very clearly and definitely set forth by Bather (1909a). The Aristotle's lantern is described as inclined, when in side view it subtends an angle of about 90°, the character of the Echinocystoida and Perischoechinoida (Plate 27, fig. 6) ; erect, when the faces of opposite sides are approximately parallel, the usual character of Recent regular Echini (Plate 5, fig. 2) ; and procumbent, when the flaring lantern rests almost on the base of the test, as in the Clypeastrina. Terms applied to the parts of the lantern are as follows. There are five teeth which are grooved or keeled (text-figs. 210, 212, p. 184). Each tooth is 30 ROBERT TRACY JACKSON ON ECHINI. embraced and held tightl}^ in place by one of the pyramids, and proximallj' the soft growing end is enveloped by the dental capsule (Plate 5, figs. 1, 6). The pyramid dorsally includes a triangular space, the foramen magnum, which is shalloiv, or deep, and dorsally open, or closed by the lateral extension of the epiphyses (text-figs. 207-21G, p. 184). The two half -pyramids, forming one pyramid and joined by suture, are inierradial in position (text-fig. 211, p. 184); or the two half-pyramids joined by an interpyramidal muscle are radial in position (text-fig. 220, p. 191). These radial half-pyramids are a feature connected with the view of considering a sea-urchin as composed of five radial parts (pp. 62, 190, text-figs. 217-220). Each half-pyramid bears on its inner face an elevated ridge, the dental slide, which supports the tooth (Plate 2, fig. 10); and has a lateral wing that extends to the oesophageal cavity; on the outer face of the wing in regular Echini are ridges for the attachment of interpyramidal muscles (Plate 5, figs. 3, 5, 9). The dorsal face of a half-pyramid, seen when the epiphysis is removed, is described as smooth when no pits exist, Perischoechinoida, Cidaroida, or is described as having pits when such structures exist, Centrechinoida (text-figs. 208-213). Each half-pyramid is surmounted by an epiphysis, which is narrow, when it extends only slightly beyond the half-pyramid, or wide, when it extends across the foramen magnum uniting in median suture with its fellow of the opposite side (text-figs. 207-216, p. 184). Each epiphysis presents a glenoid cavity and tubercles for articulation with the brace, and in the Camarodonta crests for support of the tooth (Plate 2, fig. 14; Plate 5, figs. 2, 5, 9). The brace is a block-shaped plate and presents cond;//fs and foramina which interlock with the glenoid cavities and tubercles of the epiphyses (Plate 2, figs. 9, 13, 15). In all regular Echini there are five compasses, each composed of two pieces joined by a transverse suture (Plate 2, fig. 12). The lantern muscles are termed protractors, retractors, interpyramidal, brace muscles, circular compass and radial compass muscles (Plate 5, figs. 1, 2, 4, 12; Plate 4, fig. 4; text-figs. 218-236, pp. 191, 193, 197). The perignathic girdle, when existent, consists of processes arising from the basicoronal plates. Apophyses are upward growths of basicoronal interambulacral plates; auricles are here limited to separate pieces arising from the basicoronal ambulacral plates, to which they are joined by close suture, not being continuations of these plates as apophj'ses are of inter- ambulacral plates (text-figs. 218-230, pp. 191, 193). The auricles in Phormosoma give rise to radial somatic and radial peristomal muscles, which are known only in the Echinothuriidae (text-fig. 226). The diameter of the test in regular circular Echini, where the axes are known, is measured on the plane of the ambitus and passing through ambulacrum III and interambulacrum 5. In regular Echini, which are elliptical in horizontal outline (Echinometridae), or elongate irregular Echini, the length is taken in the longer axis and includes the over-all measurement of the corona. PART I. COMPARATIVE MORPHOLOGY OF ECHINI. Having enjoyed the opportunity of studying most of the genera and species of Palaeozoic Echini, it is of importance to sum up the comparative structure and morphology of these types and to show their relations to living forms. Palaeozoology is only one aspect of zoology, distinguished from the study of living animals about as embryology, a study of the young, is in so far separated from a study of adult organ- isms. As Huxley said, the only difference between a collection of fossils and a collection of recent animals is that one has been dead longer than the other. For an intelligent study of fossils their living representatives must be constantly borne in mind, as their details of struc- ture throw a flood of light on fossil forms. Conversely a study of fossils in relation to the living gives a knowledge of the earlier and often primitive representatives of a group, throws light on the structure of the adult, and especially of the young of living types, and gives a rounding out of the knowledge of a group that cannot be attained by a si-udy of the living forms alone. This is my excuse, if such is necessary, for including in this discussion representatives of living and other post-Palaeozoic Echini in the summing up of the relations and structure of the Pal- aeozoic types, with which this memoir is primarily concerned. Under the consideration of Morphology are taken in order the form of the test, orienta- tion, the pentamerous system and variation, the structure of the skeleton, and growth. In the sea-urchin there are many anatomical parts for consideration, and they are taken up in the following order: the ambulacrum and interambulacrum of the corona, basicoronal plates, imbrication, spines, peristome, ocular and genital plates, periproct, Aristotle's lantern, and perignathic girdle. Form of the Test. The general shape of the test of a sea-urchin varies greatly, but is more constant in Palaeo- zoic than in post-Palaeozoic types. The test may be elliptical with the vertical axis somewhat longer than the horizontal through the mid-zone, Bothriocidaris, Palaeechinus ellipticus (Plate 29, fig. 2), Lepidesthes colletti (Plate 69, figs. 2-6). In Recent Echini the test is occasionally elliptical, as in Amblypneustes formosus Valentin, a species in which the height may exceed the diameter, though not always (A. Agassiz, 1873, p. 479). The test may be nearly or (31) 32 ROBERT TRACY JACKSON OX ECHIXT. quite spherical, Maccoya sphaerica (Plate 32, fig. 5), most species of Lovenechinus and Melon- echinus (Plate 55); or depressed spheroidal, as in Archaeocidaris and Cidaris. In all the above types the ambitus coincides very nearly with the mid-zone. The hori- zontal section in any zone may be nearly or quite circular, or modified by more or less pro- nounced melon-like ribs or elevations of both the ambulacra and the interambulacra, as in Melonechinus (Plate 60, fig. 3). The test may be depressed spheroidal with ambitus below the mid-zone, as in Lepidocentrotus ivhitfieldi (Plate 19, figs. 6, 7), or recent Strongylocentrotus. Rarely the ambitus is above the mid-zone, as in the high obovoid Lepidesthes corcyi (Plate 66, figs. 8-10) and Melonechinus obovatus (Plate 54, fig. 2). As far as I am aware, this character of ambitus above the mid-zone is known otherwise typically only in the recent depressed Echino- strephus molare (Blainville), in which Mr. Agassiz (1873, p. 457, Plate 5a, fig. 11) showed that the greatest diameter is near the abactinal surface. It may occur occasionally as a variation, as I found in three specimens of Strongylocentrotus purpuratus out of 120, and in one specimen of Strongylocentrotus drohachiensis in 33,000. It is undoubtedly an exceptional character for the ambitus to be above the mid-zone in fossil or Recent Echini. The test may be depressed with strongly pentagonal outline, as in Hyattechinus pentagonus (Plate 24, figs. 1-4). It may be circular but greatly flattened dorso-ventrally, as in Hyattechinus rarispinus (Plate 22), which apparently closely approached Echinarachnius in form. Or finally, in Palaeozoic types, the test may be flat on the base with rounded dorsal side, of moderate bilaterality, almost Cly- peaster-like in form, as in Hyattechinus beecheri (Plate 24, figs. 5-8). This includes all the forms of tests known in the Palaeozoic species. The test may be radially equal with central periproct, as are most Palaeozoic Echini and most post-Palaeozoic regular Echini except Echinometra and allies. Or it may be bilateral with central periproct, Hyattechinus beecheri (Plate 24, figs. 5-8; Plate 25, fig. 5), a character not known to me in any other Echini. Or it may be more or less regular in form but with the periproct eccentric in an interambulacrum, which is probably the odd posterior interambu- lacrum, Echinocystites. Orientation. The matter of orientation of a sea-urchin in relation to its true axis is important, and has been briefly touched upon in the Introduction. Loven (1874) urged that the proper orienta- tion of an echinoid is to take as the antero-posterior axis a line drawn through an ambulacrum and opposite interambulacrum, in such a plane that the madreporite lies in the right anterior interambulacrum. It seems rather arbitrary in the regular modern Echini at first sight, but there are good reasons for the acceptance of this orientation here. The most obvious reason, as pointed out by Loven, is that it is morphologically the same line on which bilaterality is ORIENTATION. 33 developed in all the irregular Echini. The ambulacra fall into a posterior pair, the bivium of Loven, and an anterior set of three, consisting of the right and left anterior ambulacra and the odd anterior ambulacrum, the trivium of Loven. It has been claimed that in regular Echini the madreporite is the only structure on which orientation could be based (A. Agassiz, 1881, p. 7). Wliile this is apparently true in many Echini, I have attempted to show under the discussion of ocular plates that in many regular modern Echini a bilateral symmetry is expressed and orientatioii obtained by the order in which ocular plates reach the periproct. To state the case in brief: in the young of apparently all modern regular Echini, the ocular plates are exsert." The same condition exists in the adult of many species. On the other hand, many species have in the adult one or more plates insert, or reaching the periproct, either as a typical character or as a variation. On the basis of ob- servations on 50,000 specimens, the evidence is that the first oculars to become insert are the plates of the bivium, next the plates of the posterior pair of the trivium, and last, if at all, the anterior odd plate of the trivium. The order of reaching the periproct is I, V, or V, I, then IV, II, III. This order is very closely adhered to, as shown later in tabulated form. The ocular plates therefore in many regular Echini express a bilateral symmetry in this group, and an orientation passing through ambulacrum III and interambulacrum 5, the plane of symmetry adopted by Loven. As studied from the ventral view, Loven (1874) showed graphically that the size and character of the primordial ambulacral plates give .excellent data by which to orient a sea- urchin, at least in the Irregulares and often in the Regulares as well. He showed that of the ten basicoronal ambulacral plates in (typical) clypeastroids and spatangoids, the la, Ila, 1116, IVa, \b are larger, and in spatangoids possess two separate pairs of pores, or two separate single pores, indicating two tube-feet. On the contrary, the 16, 116, Ilia, IV6, Va are smaller, and in the spatangoids bear only a single tube-foot, Collyrites (Plate 3, fig. 15). This system of alternation has but this one combination, a sort of key, by which the axes can be ascer- tained with entire certainty, as shown in numerous genera and species (Loven, 1874). Mr. A. Agassiz shows the same characters in his splendid work on Panamic Echini in numerous spatangoids as well as in many regular Echini. Loven (1892) showed that in the primordial ambulacral plates on the peristome in young Goniocidaris (Plate 2, fig. 1) the same system of alternation of large and small plates prevails. He (1874) also showed that it exists in the primordial plates surrounding the mouth in adult 1 Mr. Agassiz (1881, p. 7) objected to Loven's orientation as it does not take account of the madreporite in relation to the spiral system of development of plates. The spiral system was first suggested by Professor L. Agassiz (1834) and is considered by A. Agassiz in several publications (1864, p. 12; 1874, pp. 640, 722, 724; 1881, p. 7; 1892, p. 95). I regret that I have not succeeded in recognizing the spiral arrangement of plates in Echini. 2 It is possible that in Aspidodiadema and close allies the oculars are insert from their earliest stages. 34 ROBERT TRACY JACKSON ON ECHINI. Cidaris. Mr. A. Agassiz (1904) in the Panainic Echini shows the same system in young Poro- cidaris cobosi A. Ag. (his Plate 12, fig. 2), young Salenocidaris miliaris A. Ag. (his Plate 16, fig. 1), young Phormosoma placenta A. Ag. (my Plate 3, fig. 10). He also shows the interesting fact that the same order of ambulacral plates exists around the mouth in the adults of Cidaris (Dorocidaris) panamensis (A. Ag.) (his Plate 3, fig. 4), Porocidaris. milleri A. Ag. (his Plate 7, fig. 6), Echinosonia hispidum (A. Ag.) (his Plate 41), Phormosoma zeylandiae A. Ag. (his Plate 50, fig. 1), and Kamptosoma indistinctum A. Ag. (his Plate 50, fig. 1). It is true that in the adult of most of the regular Echini, where the primordial ambulacral plates surround the mouth, usually more or less isolated, they have not retained the relative proportion of the Loven system seen in the young, and from them one could not orient a sea- urchin. This has been dwelt upon at some length because in my studies of Bothriocidaris archaica (Plate 1, fig. 1) it was found that the ambulacral plates around the mouth are arranged according to the Loven law, which shows that it already was at work in the oldest known sea- urchin. It is worth mentioning that I was ignorant of the law at the time the drawing was made so that no prejudice came in to influence the making of the drawing. Loven maintained that the sea-urchin could also be oriented by the relative size and posi- tion of the ten interambulacral plates found at the base of the corona in regular Echini, the la, 2a, 36, 4a, 5b plates being smaller, the lb, 2b, 3a, 46, 5a larger. This is often true, but often not true, and it seems that the law has no general application. Loven's laws of orientation were carefully studied to see if correct orientation could thus be obtained in the Palaeozoic types, but with the exception of Bothriocidaris without success. In Palaeozoic genera, as far as known, ambulacral plates always extend on to the actinostome as far as the mouth, as shown in Bothriocidaris, Hyattechinus, Pholidechinus, Melonechinus, and Lepidesthes, and if suffi- ciently well preserved, the orientation by primordial ambulacral plates might be applied, as in Bothriocidaris. This, however, must await future study. In the cases of Lepidesthes formosa (Plate 68, fig. 5), L. colletti (Plate 71, fig. 1), Meek- echinus elegans (Plate 76, figs. 1, 6), and Lovenechinus lacazei (text-fig. 240), the specimens are correctly oriented by the presence of a madreporite. Bothriocidaris archaica (Plate 1, fig. 1) is oriented by the relation of the primordial ambulacral plates; Bothriocidaris pahleni and B. globulus (Plate 1, figs. 6, 9) by the large ocular plate which, as shown in B. archaica, overlies ambulacrum III. Hyattechinus beecheri (Plate 24, figs. 5-8) is oriented bj^ the plane of bilateral symmetry through an ambulacrum and jjosterior interambulacrum. Echinocystites pomum (Plate 20, fig. 1) is oriented by the eccentric anal area situated in an interambu- lacrum, presumably the odd posterior. The other species of Palaeozoic Echini figured cannot strictly be said to be oriented, as the letters designating areas are selected arbitrarily in each specimen without regard to a definite axis, which is an unknown quantity. The letters are used simply as a convenience in description for reference to specific areas. THE PENTAMEROUS SYSTEM AND VARIATION. 35 The Pentamerous System and Variation. In echinoderms as a whole the pentamerous system is remarkably constant as a character. In cystoids, however, we may have less than five ambulacra, as in Echinosphaerites, which has three ambulacra, or more than five, as in Caryocrinus, which has many ambulacral parts. In starfishes many genera, as Heliaster and Solaster, have typically more than five areas, and in some that have five, variation is common. In brittlestars more than five areas occur rarely as a specific, and in one instance as a sexual character. In all other groups of echinoderms the pentamerous system is very constant in all genera, and is departed from only as an indi- vidual variation. This variation is quite common in blastoids and crinoids. In Echini amongst Palaeozoic species, no case is known of departure from the pentamerous system, and it is apparently rare in living Echini. Bateson (1894) gives thirteen cases of complete or partial reduction to four-rayed sea-urchins in several genera of post-Palaeozoic fossil or Recent Echini. He also gives two cases of six-rayed urchins, a Galerites and an Amblypneustes, and three cases in which there is a sixth ambulacrum only. Chadwick (1898) described a four- rayed Echinus esculentus Linne, in which, however, there were five teeth. Ritchie and Mcintosh (1908) describe, very carefully, an Echinus esculentus with a partial reduction to four areas. Tower (1901) records an Echinarachnius parma from Woods Hole with one of the ambulacra imperfectly developed, and Hawkins (1909a) an Amblypneustes with two ambu- lacra incomplete dorsally. Osborn (1898) described a four-rayed Arbacia pun£tulata, de Loriol (1883) a partially six-rayed specimen of Stomopneustes, and Ribaucourt (1908) a completely hexamerous Strongylocentrotus lividus. I have been fortunate enough to study 71 specimens of Echini with a complete or partial departure from the pentamerous system. Besides being curiosities, these show very interest- ing morphological characters. Sixty were discovered in an examination of about 50,000 specimens for the characters of the apical disc, so that the variants from the pentamerous system averaged a httle more than one to a thousand. The variants are partially or completely trimerous, tetramerous, and hexamerous. They are numerically as follows: one Eucidaris tribuloides in 849 specimens, six Arbacia punctulata in 2,329 specimens, two Echinus magel- lanicus in 200 specimens, 38 Strongyloeentrotus drobachiensis in 33,000 specimens, one Strongy- locentrotus eurythrogrammus in 56 specimens, two Toxopneustes variegatus in 1,043 specimens, five Toxopneustes atlanticus in 2,643 specimens, three Tripneustes esculentus in 703 specimens, one Echinometra lucunter in 754 specimens, and one Colobocentrotus atratus in 82 specimens. As later discussed, the ocular plates seem to exert a controlling influence in the building up of the corona, as below and in immediate contact with the oculars originate the coronal plates, both ambulacral and interambulacral. In connection with each ocular is developed a whole ambulacrum, and, in addition, a half-interambulacrum on either side. That is, while 36 ROBERT TRACY JACKSON ON ECHINI. an ambulacrum originates on the ventral border of an ocular, each interambulacrum may be considered as composed of two halves, the plates of which originated on the left or right of the area in contact with the adjacent oculars (text-figs. 217, 218, p. 191). If this is true, then the loss of an ocular would cause a failure to develop of the plates that normalh' went with it, also an abnormal position of an ocular (Plate 7, fig. 2) should cause an abnormal distribution of the associated coronal plates. The cases of departure from the pentamerous system are taken up in the order of their structural characters. They are described in at least one case of each class and other cases are noted. The cases fall under twenty more or less distinct combinations of characters, and to facilitate the distinctions, they are numbered under so many distinct heads. Trimerous. 1. Three ambulacra, inter ambulacra, oculars, and genitals, four teeth. — A specimen of Strongylocentrotus drobachiensis collected at Dumpling Islands, North Haven, Maine, measur- ing 5.5 mm. in diameter (R. T. J. Coll., 812), is the nearest approach to a completely trimerous sea-urchin known. There are three ambulacra and interamlaulacra throughout the corona. There are also three oculars and genitals. The oculars all reach the periproct, an extraordinary character in the species, but no rules can be expected to apply in such a freak. There are four teeth and eight primordial ambulacral plates in the peristome. 2. Pentamerous ventrally, three ambulacra and five interambulacra dorsaUy, five oculars and genitals. — It is quite a frequent occurrence for one ambulacrum to fail to reach the apical disc, but rarer for two ambulacral areas to drop out. In a specimen of Strongylocentrotus drobach- iensis from Dumpling Islands, 43 mm. in diameter (R. T. J. Coll., 813), the test is quite normal ventrally, but dorsally ambulacra III and IV drop out (similarly, but later than in Plate 7, fig. 4), so that interambulacra 2, 3, 4 come in contact dorsally and make a continuous series of plates. All five interambulacra continue to the apical disc. There are apparently five oculars, but III and IV are imperforate and merged in a series of split genitals. There are five genitals, but genital 3 is imperforate and much split up by secondary sutures. A Strongy- locentrotus drobachiensis from Frenchman's Bay, Maine (R. T. J. Coll., 907), which measures 53 mm. in diameter, is pentamerous ventrally but ambulacra I and V drop out below the mid- zone. All oculars are in place but I and V are imperforate. As a result of this structure interambulacra 1, 5, 4 are confluent dorsally and the six columns of plates of these three areas extend to oculars II, I, V, IV as usual. A specimen of Toxopneustes atlanticus from Bermuda (R. T. J. Coll., 814) is quite similar. It is 52 mm. in diameter and ventrally normally pentamerous. Ambulacra III and IV fail to reach the apical disc by a distance of 10 mm., so that interambulacra 2, 3, 4 are in contact THE PENTAMEROUS SYSTEM AND VARIATION. 37 dorsally. There are five oculars and genitals, but oculars III and IV are imperforate. An imperforate ocular is usual when an ambulacrum drops out (Plate 7, fig. 4) and evidently is a correlated structure. The same structure is shown also in a specimen of Arbacia pundulata from Woods Hole, Massachusetts (R. T. J. Coll., 815). It is a small individual, 22 mm. in diameter, and distorted. Pentamerous ventrally, the ambulacra I and IV drop out dorsally so that interambulacra 5, 1, and 3, 4 are in contact dorsally. Five oculars and genitals are in place dorsally but somewhat distorted. There are as an independent variation eight plates in the periproct of this specimen. 3. Pentamerous ventrally, three ambulacra and interamhulacra dorsally, three oculars and five genitals in place, two oculars out of place. — An ocular or a genital may be wanting, but when present, they are almost universally in place in the apical disc. No case of a disjunct genital Text-fig. 1.— Slrongijlocentroliis drobachiemis (O. F. Miiller). Smllivan, Maine. Diam. 42 mm, R. T. J. Coll., 710. X 3.5. Oculars I and IV with associated coronal plates displaced. 38 ROBERT TRACY JACKSON ON ECHINI. has been seen, but in two cases oculars are separated more or less widely from the apical disc (text-fig. 1, and Plate 7, fig. 2). In a Slrongylocentrotus drobachiensis (text-fig. 1) a curiously abnormal and instructive condition occurs. The test is pentamerous ventrally, but dorsally the terminations of two ambulacra and four half-interambulacra are disjunct from the apical disc. There are five genitals and three oculars in the apical disc, but two oculars, I and IV, have become separated from the oculo-genital ring. The system is the same as that described in Toxopneustes (p. 42; Plate 7, fig. 2), only, as the separation occurred later in life, the distance of the oculars from the apical disc is not so great as in the Toxopneustes. Ocular IV (text-fig. 1), is furthest removed. .Ambulacrum IV is twisted dorsally, but extends ventrally as usual. The right half of interambulacrum 3 abuts against ocular IV, and the left half of interambulacrum 4 curls around dorsally so as to come in contact with the opposite side of ocular lY. The left half of interambulacrum 3 and right half of 4 extend to oculars III and V respectively as usual. A similar condition exists in the relation of the ambulacrum and half interambulacra to the misplaced ocular I. If the abnormality had occurred earlier in growth, this would have made a very striking specimen, but yet it elucidates the principles involved quite clearly. In this specimen it is clear that ambulacra I and IV, with the associated half interambulacra follow the oculars I, IV in their displaced position. Teframerous. 4. Four ambulacra, interambulacra, oculars, genitals, and teeth. — More or less completely four-rayed specimens are the commonest departure from the pentamerous system, yet com- pletely four-rayed specimens were seen in only nine cases, all in three species. In a specimen of Strongylocentrotus drobachiensis (Plate 6, fig. 9) there are four ambulacra, interambulacra, oculars, genitals, and teeth, and eight primordial ambulacral plates in the peristome. It is oriented by the madreporite and character of the bivium reaching the periproct, or insert, as it is here called, which is a strongly dominant feature of the species. Accepting this orienta- tion, the absent parts are ocular IV and its associated parts, which are ambulacrum IV, the right half of interambulacrum 3 and the left half of interambulacrum 4. The missing genital may be either 3 or 4, which, it is perhaps impossible to say. Four other specimens of Strongylo- centrotus drobachiensis show the same structure. In all, the bivium is insert and the missing parts are the same as in the specimen just described. One of these specimens is from Dumpling Islands, Maine (R. T. J. Coll., 816), and measures 18 mm. in diameter. Two are from French- man's Bay, Maine (R. T. J. Coll., 908, 909), and measure 42 and 52 mm. in diameter respectively. The fourth specimen is from Truro, Massachusetts (R. T. J. Coll., 817), and measures 26 mm. in diameter. In this specimen the lantern is wanting, but there are eight auricles which indicate a four-parted lantern. Four specimens of Microcyphus maculatus, THE PENTAMEROUS SYSTEM AND VARIATION. 39 from Mauritius, with complete or partially tetramerous tests, were found in 343 specimens examined in the Museum of Comparative Zoology by Dr. H. L. Clark. Three of them fall under the present head, with four teeth, eight primordial ambulacral plates, four ambulacral and interambulacral areas in the corona, four oculars and genitals. In one of the specimens two of the genitals are fused, and in one of them a genital is split into three parts. An Echinometm lucunter from Jamaica, in the Museum of Comparative Zoology, is com- pletely four-rayed, with four teeth, eight primordial ambulacral plates, four ambulacra and interambulacra in the corona, four oculars and genitals. The absent parts are ocular and ambulacrum I and genital 1, the left half of interambulacrum 1 and the right half of interambulacrum 5. 5. Four ambulacra, interambulacra, and oculars, five genitals, four teeth. — This structure differs from the last described only in that there are five instead of four genital plates. An example is seen in Arbacia punctulata (Plate 8, fig. 2; Plate 6, fig. 1). The specimen is- small, 26 mm. in diameter, but is higher than usual proportionately, 19 mm. It is perfectly shaped, not abnormal in appearance. There are five genitals, the only pentamerous part of the animal observed, but there are only four genital pores, one being absent in the madreporite. The specimen is oriented by the madreporite and planes of arrangement of the periproctal plates. Only four oculars are present. The absent parts are ocular and ambulacrum II completely, also the right half of interambulacrum 1 and the left half of interambulacrum 2. The corona consists, therefore, of four ambulacra with three interambulacra and two half-interambulacra. There are four teeth, four pairs of auricles, and eight primordial ambulacral plates. A second almost completely four-rayed Arbacia punctulata, from Woods Hole, Massachu- setts, is in the American Museum of Natural History. This like the above is small, perfectly formed, not at all distorted. It measures 25 mm. in diameter through the ambitus and 18 mm. in height. There are five genital plates, but the madreporite has no genital pore, and lies in close contact with genital 3 just as the madreporite lies in contact with genital 1 in Plate 6, fig. 1. There are four oculars, ambulacra, interambulacra, and teeth throughout. The absent parts are ocular and ambulacrum III, the right half of interambulacrum 2, and the left of interambula- crum 3 (compare Plate 6, fig. 1). A third specimen of Arbacia punctulata, from Woods Hole, in the Museum of Comparative Zoology, has a similar structure. It has four ambulacra, interambulacra, and oculars, but five genitals, one depauperate and imperforate. The wanting parts are ocular and ambulacrum I, the right half of interambulacrum 5, and the left half of interambulacrum 1. The teeth are wanting. The Arbacia punctulata described by Osborn (1898) is similar to the above three, the five genitals being the only pentamerous portion. In his specimen as gathered from the description and figure, the absent parts are ocular and ambulacrum I, the right half of inter- ambulacrum 5, and the left half of interambulacrum 1. 40 ROBERT TRACY JACKSON ON ECHINI. In Yale University Museum a specimen of Arbacia lixula from the Mediterranean has four areas. It is one of a lot of four specimens (cat. no. 199). There are five genital plates, but only four oculars and four ambulacra and interambulacra which extend from the basi- coronal row to the apical disc. The lantern is wanting. The absent parts are ocular and ambulacrum I, the right half of interambulacrum 5, and the left half of interambulacrum 1; therefore the left half of interambulacrum 5 and the right half of interambulacrum 1 are in contact and form a single area (compare Plate 6, fig. 1). This area is wider than usual for an interambulacrum, and the plates bear more primary tubercles than usual, but the tubercles are spaced about as in other areas, and in normal specimens. This indicates, as shown in the reverse condition in Tripneustes (p. 47), that tubercles and spines are distributed at a given distance apart, rather than a given number to a plate, as a species character. Genital 1 is somewhat distorted. Professor Verrill (1909) has given an excellent photographic figure of this specimen. A small specimen of Strongyloccntrotus drobachiensis (Plate 7, fig. 3) has much the same character as the above described Arbacias. The specimen is normally shaped except that it is unusually high, dome-shaped. It is 35 mm. in diameter and 24 mm. high. There are four ordinary genitals and a small plate between the madreporite and ocular I may be considered a depauperate and imperforate fifth genital. There are four oculars. The specimen is oriented by the madreporite and the two insert ocular plates which in this species mark the bivium. The absent ocular on the basis of this orientation is evidently that one which should lie l^etween the madreporite and the depauperate genital next to it. The absent part of the corona is ambulacrum II, also the right half of interambulacrum 1 and the left half of interambulacrum 2. The lantern is wanting, but there are four pairs of auricles indicating a four-parted lan- tern. A similar structure occurs in a specimen of the same species from Dumpling Islands (R. T. J. Coll., 818). It is small, 15 mm. in diameter, not distorted, and completely four-rayed except for the five genitals. The madreporite has no genital pore. The missing parts are ocular and ambulacrum II, the right half of interambulacrum 1 , and the left half of interambula- crum 2. 6. Four ambulacra {bui ten primordial ambulacral plates), four interambulacra and oculars, but five genitals and teeth. — This combination was found in a specimen of Strongylocentrotus drobachiensis from Dumpling Islands, 5.5 mm. diameter (R. T. J. Coll., 819). Being very young, all the genitals are imperforate except for the madreporic pores. Ocular I only is insert. The missing parts are ambulacrum and ocular II, the right half of interambulacrum 1, and the left half of interambulacrum 2. Surrounding the five teeth are ten primordial ambulacral plates. This indicates that it began life as a pentamerous individual, but no trace of a fifth ambulacrum is seen in the corona. A second specimen with these characters occurs in an Echinus magellanicus, from Port THE PENTAMEROITS SYSTEM AND VARIATION. 41 Gallant, Patagonia, in the Museum of Comparative Zoology. It has five teeth and ten pri- mordial ambulacral plates, four ambulacra and interambulacra in the corona, four oculars, but five genitals. The wanting parts are ocular and ambulacrum V, the right half of inter- ambulacrum 4 and the left half of interambulacrum 5. 7. Four ambulacra, hut five interambulacra, oculars, genitals, and teeth. — In Arbacia ■punctulata (Plate 7, fig. 1) an ambulacrum is entirely absent, but otherwise the specimen is pentamerous. It is a small specimen, 22 mm. in diameter. Dorsally there are five genitals and oculars, ventrally five teeth and ten primordial ambulacral plates. There is no trace of ambulacrum IV in the corona. Interambulacra 3 and 4 have each two columns of plates which extend dorsally to oculars III, IV, V, as usual in columns of these two areas. In addition to the four columns, there are ventrally several plates representing a fifth column and one plate a sixth column. These extra columns cannot be referred with a,ssurance to either areas 3 or 4, but were in all probability developed below ocular IV and serve as space fillers in the corona to occupy the place that would have been filled by ambulacrum IV if it had developed. This specimen superficially is suggestively like Palaeozoic Echini where four or more columns exist in an interambulacrum, the median columns being made up of plates which in general are hexagonal, and the adambulacrals pentagonal. Here, however, the many columns are the result of the union of two areas, not one area as in the Palaeozoic. 8. Five ambulacra, four interambulacra, four oculars {one with two pores), four genitals, five teeth. — A very peculiar case is the Strongylocentrotus drobachiensis, Plate 7, fig. 5. The specimen is good sized, 50 mm. in diameter and 30 mm. high. It has a pronounced bulging on the median suture line of ambulacra I, II. There are four genitals and four oculars. One of the oculars has two pores, indicating the termini of the radial water tubes of the two sub- jacent areas. There are five ambulacra, but interambulacrum 1 is entirely wanting. The ambulacra I, II have each two columns of plates, but the 16 and Ila are very narrow columns. It is worth noting that on the line of sutural contact of these two areas the outer pore of each pore-pair lies above the inner one, the usual condition where an ambulacrum abuts on an interambulacrum. There are five teeth and five pairs of auricles. A second specimen showing this same structure is a Toxopneustes atlanticus, from Bermuda, R. T. J. Coll., 895. It measures 62 mm. in diameter, and is not distorted. There are four genitals and oculars, but one of the oculars has two pores, as in Plate 7, fig. 5, indicating that the pores connected with the two associated ambulacral areas. There are five ambulacra, IV and V being in contact throughout, as interambulacrum 4, also genital 4, are entirely wanting. There are five teeth, but only eight gill cuts. 9. Five ambulacra, four interambulacra, five oculars, four genitals, five teeth. — • A Toxo- pneustes variegatus from Boca Ciega Bay, Florida (R. T. J. Coll., 820), 52 mm. in diameter, has five ambulacra, but interambulacrum 5 and genital 5 are wanting. A Strongylocentrotus 42 ROBERT TRACY JACKSON ON ECHINI. drobachiensis from Truro, Massachusetts (R. T. J. Coll., 821), 32 mm. in diameter, is similar, with five ambulacra, but interambulacrum and genital 5 are entirely wanting. Both speci- mens have five teeth. The condition in both is similar to Plate 7, fig. 5, excepting that there are two oculars covering the adjacent ambulacra V and I instead of one ocular with two pores. A third specimen showing this combination is a Microcyphus maculatus from Mauritius, in the Museum of Comparative Zoology. In this there are five teeth, ten primordial ambulacral plates, five ambulacra and oculars, but four interambulacra and genitals. Interambulacrum and genital 5 are absent, so that oculars and ambulacra V, I are in contact throughout. 10. Five ambulacra, four interambulacra, five oculars, genitals, and teeth. — This typo like the last has an interambulacrum wanting, but differs in that there are five instead of four geni- tals. A Strongylocenlrotus drobachiensis from Calderwood Island, Fox Island Thoroughfare, Maine, measuring 34 mm. in diameter (Plate 5, fig. 16), has five complete ambulacra but only four interambulacra. Interambulacrum 1 is absent and ambulacra I and II are in contact throughout their length. There are five oculars and genitals, but oculars I and II are in contact and shut out genital 1 from contact with the corona. This is the only case of a genital dorsal to the oculars seen excepting in Bothriocidaris archaica where it is a species character. Genital 1 in this case is very small and imperforate. There are five teeth, ten auricles, and ten primordial ambulacral plates. 11. Five ambulacra ventraUy, four interambulacra throughout, four ambulacra above the ventral border, four oculars and genitals, five teeth. — This peculiar structure occurs in one speci- men only, an Arbacia punctulata (R. T. J. Coll., 881), kindly sent me from Johns Hopkins Uni- versity through Dr. G. T. Hargitt. The specimen is 43 mm. in diameter, 31 mm. high, and quite symmetrical. There are five teeth and ten primordial ambulacral plates, but only eight peristomal gills. At the ventral border of the corona there are five ambulacra, but ambulacrum II exists for only a distance of 2 mm. in which there are three pore-pairs. Above this point the ambulacrum is entirely wanting. There are four interambulacra only, the area lying above ambulacrum II being a complete area, but consisting of the left half of interambula- crum 1 and the right half of interambulacrum 2 (compare Plate 7, fig. 3). Dorsally there are four oculars, genitals, and periproctal plates. Ocular II has quite disappeared. 12. Pentamerous ventraUy, four ambulacra and interambulacra dorsally, four oculars in place {one ventral), five genitals. — One of the most suggestive and interesting cases observed is the Toxopneustes variegatus, Plate 7, fig. 2. The specimen is full grown and has a strong bulge on the abnormal side. It is 61 mm. in diameter through III, 5; 67 mm. through 4, II; and 39 mm. high. Dorsally in place there are five genitals and four oculars, but just below the ambitus lies a fifth ocular which by some chance became separated and left behind during the dorsal growth of the sea-urchin. As there are the usual five areas ventraUy, the specimen is readily oriented by the position of the madreporite. Ocular II, which should be between THE PExNTAMEROUS SYSTEM AND VARIATION. 43 genitals 1 and 2, has in its abnormal position grown to a large and somewhat irregularly shaped plate (compare oculars in text-fig. 1, p. 37). Below this ocular extends ambulacrum II, which finds its dorsal limit in this plate. Also extending ventrally from ocular II there are two half-interambulacra, the right half of interambulacrum 1 and the left half of interambula- crum 2. Proceeding dorsally from ocular II, the coronal plates that are typically associated with this plate cease, but the left half of interambulacrum 1 and the right half of interambula- crum 2 fill the space and extend respectively to ocular I and ocular III, their normal point of origin. There are five teeth. This specimen owes its peculiarities to relatively late influences, not congenital. It is similar in principle to text-fig. 1, p. 37, but is more striking as the ocular is farther removed from its normal position in the apical disc. It lends weight to the view held that ocular plates are a controlling influence in the development of the corona, and that from each ocular originates an ambulacrum and two half-interambulacra (compare text- fig. 218, p. 191). 13. Pentamerous ventrally , four ambulacra and interambulacra dorsally, four oculars, and five genitals. — kStrongylocentrotus drdbachiensis from York, Maine (R. T. J. Coll., 823), 40 mm. in diameter, is pentamerous ventrally, but ambulacrum II consists of only a pair of plates in the basicoronal row. The associated two half-interambulacra of areas 1 and 2 are similarly highly reduced. Above this extreme ventral area there are only four ambulacra and inter- ambulacra. The ocular associated with ambulacrum II has quite disappeared, probably by resorption. Four oculars and five genitals are in place in the apical disc. Genital 1 is depauperate and lies against the madreporite. There are five pairs of auricles. A similar structure is shown in an Echinus magellanicus from the Straits of Magellan in the collections of the United States National Museum. There are five teeth and ten primordial ambu- lacral plates. In the corona there are five ambulacra and interambulacra ventrally, but ambulacrum I and the associated interambulacral plates exist for only a short distance, above which there are only four areas. There are four oculars and five genitals of which genitals 1 and 5 are depauperate and imperforate. 14. Pentamerous ventrally, four ambulacra and interambulacra dorsally, four oculars and genitals. — A Strongylocentrotus drobachiensis from Dumpling Islands, Maine, (R. T. J. C-oll., 824), 18 mm. in diameter, is completely pentamerous ventrally. A slight distance above the ventral border ambulacrum V drops out and also the two associated half-interambulacra. Ocular V has quite disappeared. There are four oculars and genitals in place. This type differs from the last considered only in that there are four instead of five genital plates. 15. Pentamerous ventrally , four ambulacra and jive interambulacra dorsally. Jive oculars and genitals. — This is the commonest type of departure from the complete pentamerous system found, 17 cases having been seen. A small specimen of Strongylocentrotus drobachiensis (Plate 7, fig. 4) shows interesting features. The specimen is 17 mm. in diameter, 9 mm. high, 44 ROBERT TRACY JACKSON ON ECHINI. and misshapen on the abnormal side. \'entrally there are five ambulacra, interambulacra, and teeth, but ambulacrum II is developed for a short distance only. Dorsally there are five genitals and oculars, but ocular II is imperforate. The dorsal border of the ambulacrum is widely separated from its ocular. Ocular II lies dorsally and produces its share of interam- bulacral plates, although ambulacral plates ceased to be built. Columns 1 and 2 of interam- bulacrum 1 extend dorsally and abut against oculars I and II as usual. In interambulacrum 2, columns 1 and 2 extend dorsally, but only 2 reaches the apical disc, column 1 dropping out, so that in the last three rows or zones of growth there is only a single column of plates in this area, a condition somewhat comparable to that seen in area 4 of Arbacia (Plate 4, fig. 11) where also a single column of plates exists dorsally. Just above ambulacrum II in this speci- men (Plate 7, fig. 4) there are extra interambulacral plates that cannot be definitely referred to either areas 1 or 2. They are virtually space fillers, which, in my opinion, is what the interambulacral plates are. This specimen is structurally almost identical with the interesting Echinus esculentus described by Ritchie and Mcintosh (1908). In both, the ocular remained in place dorsally but imperforate, the ambulacrum existed ventrally only, and the adjacent interambulacra developed in a comparable manner, though with certain differences. In the Echinus only the four columns of the two interambulacra exist above the imperfect ambulacrum, and extend dorsally to their respective three ocular plates. A Strongylocentrotus drobachiensis (Plate 6, fig. 6) has five oculars and genitals as usual; there are five interambulacra continuous to the apical disc, but one of the five ambulacra, IV, dies out dorsally, so that interambulacra 3 and 4 come in contact and alone touch ocular IV. The distance of separation of the ambulacrum from its ocular is only slight, but it repre- sents a zone in which there are only four ambulacra. In this specimen ocular III is split, a rare variation, and genital 5 is divided. A few pores in the dorsal part of interambulacnun 3 are apparently supernumerary genital pores that lie below genital 3. An Arbacia punclulata, from Woods Hole, Massachusetts, in the American Museum of Natural History, is peculiar in that ambulacrum III fails to reach the apical disc so that there are only four ambulacra in that region. The structure is almost the same as in Plate 6, fig. 6, except that a different ambulacrum is involved. Six specimens of Strongylocentrotus col- lected at Dumpling Islands, Maine, (R. T. J. Coll., 825-830), one from Calderwood Island, Maine (R. T. J. Coll., 845), one from Frenchman's Bay, Maine (R. T. J. Coll., 846), and two from Friday Harbor, Puget Sound (R. T. J. Coll., 903, 904), a Strongylocentrotus eunjthrogravimus from New South Wales (R. T. J. Coll., 905), also a Tripneustes esculentus from Hayti (R. T. J. Coll., 831), and two specimens of Toxopneustes atlanticus from Bermuda (R. T. J. Coll., 894, 897), all show a similar structure to Plate 7, fig. 4, with five genitals and oculars in place and an ambulacrum dropping out at a greater or less distance from the apical disc. In all but one of these cases the ocular lying above the imperfect ambulacrum is imperforate. In the THE PENTAMEROUS SYSTEM AND VARIATION. 45 several cases, each of the five ambulacra is involved, but IV appears to be more frequently imperfect than any other area. 16. Pentamerous ventrally, four ambulacra and five interanibulacra dorsally, four oculars, five genitals. — This type is similar to that of 13 in that the ocular associated with an area has disappeared, but differs in that there are five interambulacra. A Strongylocentrotus from Calderwood Island, Maine (R. T. J. Coll., 832), 46 mm. in diameter, is completely pentamerous ventrally, but ocular II is wanting, and ambulacrum II falls short of the apical disc by 10 mm. The two bordering interambulacra, 1 and 2, are in contact dorsally. The dorsal plates of column 1, in interambulacrum 2 and of column 2, in interambulacrum 1 are large, not small as is usual in dorsal plates, indicating that no new ones have been recently added, which is in accord with the absence of ocular II, from which the plates of these two columns take origin. Three other specimens of the same species from Calderwood Island (R. T. J. Coll., 833-835) and a Colobocentroius atratus from the Hawaiian Islands, in the State Museum at Albany, New York, have a similar structure. In all, an ocular is absent, and the corresponding ambulacrum falls short of the apical disc, but the associated interambulacra extend to the apical disc simi- larly to the specimen just described. In the Colobocentrotus noted, ocular I is absent, and ambulacrum I falls short of the apical disc. 17. Pentamerous ventrally and throughout the corona, five oculars, but four genitals. — This line of departure from the pentamerous system is where only one genital is absent. It is a typical condition in spatangoids where genital 5 is always absent, but occurs only as an aber- rant variation in regular Echini. In Eucidaris tribuloides (text-fig. 185, p. 167) there are only four genital plates, 1 being absent. Oculars I and II are in contact, and as a result interambulacrum 1 comes in contact with two oculars as it does in Bothriocidaris archaica and in the posterior interambulacrum in spatangoids. This interambulacrum is also much narrowed dorsally and at one point a single plate fills the area laterally. A more striking case is the Arbacia punctulata shown in Plate 4, fig. 11. In this remarkable specimen there are five oculars but only four genitals. Genital 4 is absent so that oculars IV and V are in contact. The interambulacrum 4 dorsally drops out to a single column of plates; the single column and the ocular contact of the dorsal plate of the same as a regressive variation is an exact repetition of the character seen in Bothriocidaris archaica (Plate 1, fig. 2). A quite comparable case is seen in Strongylocentrotus drobachiensis (Plate 6, figs. 7, 8). In this specimen there are five oculars, of which I and II reach the periproct and V, IV, III are exsert. There are only four genitals, number 4 being absent, so that oculars IV and V are in contact. These plates are peculiar in that they are produced ventrally in an elongate fashion. Interambulacrum 4 ventrally consists of two columns of plates as usual; above the mid-zone it narrows, and the two columns give place to a single column of plates for three rows, that is, 46 ROBERT TRACY JACKSON OX ECHINI. three plates (Plate 6, fig. 8). Above this point two columns are built again for a short distance, there being at this point three plates in column 1 , and two in column 2. Proceeding dorsally, a change again occurs, and two single plates are built; the last is very high and narrow, unlike anything seen in any other echinoid. This structure is very difficult to see on the exterior of the test, but on the interior (Plate 6, fig. 8) it is perfectly plain. The dropping out to a single column is comparable to the typical character of Bothriocidaris and to the peculiar Arbacia (Plate 4, fig. 11). Dorsally, interambulacrum 4 does not reach the ocular plates, but is separated for a short distance in which ambulacra IV and V come in contact and alone reach oculars IV and V. Another case was found in an Ai-bacia punctulala from Florida (R. T. J. Coll., 889, Plate 4, fig. 12). The specimen is 33 mm. in diameter. It is pentamerous throughout except for the absence of genital 4, which is wanting, as in Plate 4, fig. 11. It differs from that specimen, however, in that interambulacrum 4 is completely developed, its two columns extending, as usual, to the apical disc, but abutting against oculars IV and V without any contact with a genital as that plate is absent. Again, a similar structure is shown in a specimen of Toxo- pneustes atlanticus from Bermuda (R. T. J. Coll., 898). This specimen measures 53 mm. in diameter. It is quite pentamerous throughout except that genital 4 is wanting. Oculars IV and V are therefore in contact and cover completely ambulacra IV and V and interambula- crum 4 as in the Arbacia, Plate 4, fig. 12. In this Toxopneustes there are two columns of plates in contact with the two oculars, but the plates are small. These several cases demon- strate that the absence of a genital does not cause a loss of the corresponding interambulacral area. Hexamerous. 18. Six ambulacra, interambulacra, oculars, genitals, and teeth. — In a fine large Tripneustes esculentus from Pernambuco, Brazil (Plate 6, figs. 2, 3) there are six areas throughout, a condition hitherto definitely known only in Strongylocentrotus Uvidus. The specimen measures 110 mm. in all tliameters as it is perfectly circular in outline. It is 58 mm. in height. In this species, when two oculars are insert, they are the bivium (see tabulation of 703 speci- mens, p. 161), so that the specimen is oriented on the basis of the insert bivium and position of the madreporite. Obviously the additional ambulacrum, ocular, and interambulacrum lie between interambulacrum 3 and ambulacrum IV. There are four normal genitals, but two, numbers 3 and 6, are fused into a single plate, a somewhat rare condition, but similar cases are seen in Strongylocentrotus and Tripneustes (text-figs. 195, 196, p. 169). There are six oculars, the sixth lying between the tips of genitals 3, 6. The ambulacra are all normal in appear- ance and at the mid-zone measure 28 mm. in width, except V, which is 29 mm. The width, 29 mm., is the same that is found in the ambulacra of a normal five-rayed specimen measuring * THE PENTAMEROUS SYSTEM AND VARIATION. 47 the same diameter of 110 mm. The interambulacra are normal in appearance superficially, but at the mid-zone they all measure 27 mm. in width, that is, they are narrower than the ambulacra. In a normal specimen of the same size the five interambulacra measure each 38 mm. in width when the ambulacra measure 29 mm. In the six-rayed specimen, therefore, evi- dently the space gained to add the extra ambulacrum and interambulacrum is attained by building ambulacra of practically the usual width, but narrowing all the interambulacra equally to much less than the usual width. This emphasizes the conclusion gathered from normal Echini that the interambulacrum is essentially a space filler and adapts itself to fill what space is available between the ambulacra which are the more essential structures. There are twelve cuts in the test ventrally for the peristomal gills, and the lantern has six teeth and all its other parts in sixes or multiples of the same. In the narrowed interambulacral areas the tubercles (Plate 6, fig. 3) occupy the same distance apart as in ordinary pentamerous speci- mens, but owing to the narrowness of the areas there are fewer tubercles on each plate. This case with the Arbacia lixula previously considered (p. 40) demonstrates that the distance apart and size of tubercles is the species feature and not the actual number of tubercles on a given plate. The same hexamerous structure as just described occurs in a Strongyloccntrotus drobachien- sis from Dumpling Islands, North Haven, Maine (R. T. J. Coll., 838, Plate 8, fig. 3). This choice specimen measures 53 mm. in diameter, and 25 mm. in height. There are six ambulacra and interambulacra complete, six oculars and six genitals, but genitals 3, 6 are fused, with ocular VI between them exactly as in Plate 6, fig. 2. The ambulacra at the mid-zone measure 11 mm. in width and the interambulacra 12 mm. In a normal pentamerous specimen of the same size the ambulacra measure 12 mm. and interambulacra 19 mm. in width. It is clear, therefore, that as in the Tripneustes, the space for the sixth areas is attained mainly bj' narrowing the interambulacra and retaining ambulacra of practically the usual width in all areas. Ventrally there are twelve cuts for the peristomal gills. Unfortunately the jaws are absent, but they were doubtless hexamerous as all other parts of the sea-urchin have this arrangement. I was startled by finding this same structure for a third time in a small Strongijlocentrotus drobachiensis, also collected at Dumpling Islands (Plate 7, figs. 7, 8). The specimen is 14 mm. in diameter and 6 mm. in height; it is perfectly shaped without any dis- tortion. There are six ambulacra and interambulacra throughout the corona, six oculars and genitals, but as in the two other cases, genitals 3, 6 are fused, with ocular VI between them, as seen in Plate 7, fig. 7. It is certainly most extraordinary that this parallel structure should exist in three specimens, and indicates what I have elsewhere pointed out, how very definite even extremely rare variation may be. Ventrally there are six teeth, twelve pri- mordial ambulacral plates around the mouth and twelve cuts for the peristomal gills (Plate 7, fig. 8). The three similar specimens of hexamerous Tripneustes and Strongylocentrotus just 48 ROBERT TRACY JACKSON ON ECHINI. described were evidently cases of congenital variation, whereas the next type considered is a variation to six parts taken on at a comparatively late stage in development. The specimen of Strongylocentrotus lividus described by Ribaucourt (1908) would belong in this series, as he says that it is completely hexamerous, having six teeth, six ambulacral and interambulacral areas, six oculars and genitals. It would be interesting to know in which area the modification takes place and if there is a fusion of genitals as in the specimens above described. 19. Pentamerous ventrally, six ambulacra and interamhulacra dorsally, six oculars, five genitals. — A most interesting condition occurs in a specimen of Tripneustes esculentus found in material kindly collected for me in Jamaica by Dr. Thomas Barbour. Tliis choice speci- men (Plate 6, fig. 4) measures 76 mm. through III, 5 and is 45 mm. in height; it presents a slight hump in the aberrant area. There are six oculars, but only five genitals. The addi- tional sixth ocular lies between ocular II and the madreporite. Ventrally the specimen is pentamerous and quite normal, but above the mid-zone a sixth ambulacrum appears interca- lated between ambulacrum II and interambulacrum 2. It extends dorsally, the two columns of plates reaching ocular VI in the usual fashion. About halfway, up the sixth ambulacrum a sixth interambulacrum appears. It consists of a single column of narrow plates which extend to oculars II and VI. The single column and ocular contact are comparable to the character seen in Bothriocidaris archaica (Plate 1, fig. 2). I believe it and the next described case are the only known instances since Ordovician times of an interambulacrum consisting of a single column of plates throughout its length. At two points the single column of interambulacral plates is separated dorso-ventrally, and at these breaks ambulacra II and VI are in contact for a brief space. Genital 3 is split horizontally, as in text-fig. 190. A like partially hexamerous condition exists in a Strongylocentrotus drobachiensis from Truro, Massachusetts (R. T. J. Coll., 836), 35 mm. in diameter. It is quite normal ventrally, but dorsally there are, as in the above case, five genitals and six oculars. Ocular VI lies between ocular V and genital 5 and is fused with the latter. A sixth ambulacrum for a length of 10 mm. lies between ambulacrum V and interambulacrum 5, the condition being as in Plate 6, fig. 4. A sixth interambulacrum, consisting of two plates in a linear series, lies between ambulacra V and VI as in the same figure. A small and distorted specimen of Arbacia punctulata (R. T. J. Coll., 882) kindly sent me from Johns Hopkins University through Dr. G. T. Hargitt, again shows a similar structure. The specimen measures 3G mm. in diameter through 4, I and 19 mm. in height. Pentamerous ventrally, dorsally a sixth ambulacrum exists for a distance of 6 mm. The additional area lies between interambulacrum 1 and ambulacrum II (compare Plate 6, fig. 4). A sixth inter- ambulacrum consisting of two plates in a vertical series lies between ambulacrum VI and ambulacrum II, as in Plate 6, fig. 4. There are six oculars and five genitals, but the latter are THE PENTAMEIIOUS SYSTEM AND VARIATION. 49 confused by secondary sutures. There are six periproctal plates, probably a coincidence in number only as six plates in that area may occur in a perfectly pentamerous specimen (text-fig. 204, p. 175). An Echinarachnius parma, from Chelsea Beach, Massachusetts, in the collection of the Boston Society of Natural History, no. 348, is another case of a similar structure. This speci- men (Plate 8, fig. 4; Plate 7, fig. 9) is 59 mm. in diameter. It is not unusual in appearance except, for the added areas. There are five ambulacra and interambulacra ventrally and nearlj' to the ambitus where the extra ambulacrum originates. The specimen is readily oriented by the position of the periproct in 5 (Plate 8, fig. 4) and also by the proportionate size of the primordial ambulacral plates (Plate 7, fig. 9), the la, Ila, III6, IVa, V6, being the larger plates as usual, according to Loven's law. The sixth areas as seen are late in development and occur between interambulacrum 1 and ambulacrum II. In the fused oculo-genital mass there are six ocular pores, one to each area as usual. There are, however, only four genital pores. One is wanting in 5 as usual, and also in 1, which is not usual. There is, however, a pore in the aberrant interradial area 6. Ambulacrum VI originates just below the ambitus, has two columns of plates and a petal as usual, and differs only in that plates below the petal are smaller, and the petal is slightly narrower than in the other areas. Interambulacrum 6 originates slightly later than ambulacrum VI and differs from the other interambulacra only in that the plates are smaller. In this specimen the hexamerous variation was evidently not congenital, for as shown, a pentamerous condition was maintained for a time, expressed in terms of growth by the introduction of six horizontal rows or zones of ambulacral plates, as seen in area II (Plate 7, fig. 9). Above this zone the added sixth ambulacrum is introduced, and it, with the extra interambulacrum, is continued throughout succeeding growth to the apical disc. 20. Pentamerous ventrally, six ambulacra, five interambulacra dorsally, six oculars, five genitals. — This structure is shown in a specimen of Strongylocentrotus drobachiensis from York Harbor, Maine (R. T. J. Coll., 847). The specimen is somewhat flattened, 27 mm. in diameter and 13 mm. in height. There are five ambulacra and interambulacra ventrally, but within 5 mm. of the peristomal border a sixth ambulacrum appears and is extended to the apical disc. The sixth ocular and ambulacriun lie between ocular and ambulacrum I and genital and inter- ambulacrum 1. The relation is comparable to that shown in Plate 6, fig. 4, except that the two ambulacral areas are in contact, as in Plate 7, fig. 5, since there is no intervening inter- ambulacrum. The genital plates in this specimen are considerably split by secondary sutures and somewhat distorted. De Loriol (1883, Plate 4, fig. 3a) describes a partially hexamerous Stomopneustes variolaris that is referable to this type of variation. I studied the specimen in Geneva. Ventrally there are five teeth, ten primordial ambulacral plates, five ambulacra, and five interambulacra. Dorsally there are six oculars and a sixth ambulacrum, but there are only five interambulacra and five genitals. 50 ROBERT TRACY JACKSON ON ECHINI. Bateson (1894) reports two cases of "total" hexamerous Echini. One is a Galerites, in which there are six ambulacra and interambulacra as seen from below; the dorsal portion and jaws are not shown. I have-not seen Meyer's original paper where this was described. The other is a six-rayed Amblypneustes, but there is no description of the specimen. Bateson also records three specimens in which there is an accessory ambulacrum. These, with de Loriol's and Ribaucourt's cases above mentioned and the eight more or less completelj' hexamerous specimens here described, include all cases I know of in which there is an increase in parts over the typical pentamerous system in Echini. It is evidently a very rare variation. Other variations from the normal have a certain interest. Four plates in the periproct is typical of Arbacia pundulata, but as later shown, variations of fewer or more than this number are common (text-figs. 200-205, p. 175). Variations in the number of genital pores are common, also as regards the distribution of madreporic pores. The variants of genital plates, or genitals and oculars, are all discussed later. The above variations from the pentamerous symmetry can all be considered as monstrosi- ties. Variation of another kind is more common, and interesting from another point of view; that is, variation in which the whole animal, or a part, is more or less fully developed in the direct line of the differential development or evolution of the species. This in Palaeozoic Echini is marked as radial variation, which was discussed in my earlier paper (1896, p. 151), and is shown in many cases here. An example is seen in Lovenechinus missouriensis, which has five columns of interambulacral plates with often a sixth column represented by one or two plates near the mid-zone. In Plate 41, fig. 1, the sixth column is represented in areas E, G, I, but in areas A, C, there are only five columns. In the young specimen (Plate 40, fig. 1) there are five columns in four areas, but in area E the fifth column is represented by only a single plate, a great departure from the typical and a close approach to the character seen in lower species of the genus, as L. lacazei (Plate 36, figs. 1-6), which has only four columns of interambulacral plates in an area as a specific character. As shown in my earlier paper in detailed studies of Melonechinus muUiporus, a given column of interambulacral plates, especially the columns from 5 upward, may come in at vary- ing zones in different areas of the same specimen, showing considerable variation in the rate of radial development. This is shown well in the zone of introduction of the seventh column in Lovenechinus septies (Plate 45, fig. 1). The actual number of columns, or differential char- acter, may vary in ilifferent areas of the same specimen, as in Hyattechinus rarispinus (Plate 23, fig. 3). In Melonechinus muUiporus (Plate 54, fig. 5) there are nine columns in area .\, but only eight in the four other areas; in Plate 60 the specimen figure 2 has nine columns in all areas, but figure 1 has eight in all areas; Plate 55, fig. 3, has nine columns in areas C, E, and G, but the other areas are not known definitely; the specimen, Plate 55, figs. 1, 2, and Plate 57, has only eight columns in all five areas. A specimen may have but seven columns of inter- STRUCTURE OF THE SKELETON AND GROWTH. 51 ambulacral plates in an area, a rather rare variation in this species. This same variation of greater or less degree of radial differential development is shown in many species in this paper. The radial . variation in a specimen may be as great as the variation of different specimens within the limits of a single species. Variation is marked as regards the number of ocular plates that meet the periproct in Recent and some Palaeozoic Echini, as fully discussed later. Variation occurs all through Echini in all parts of the organism, and numerous cases are considered in the following pages. All the evidence goes to show that aberrant variation is relatively rare, and in almost all cases variation in Echini is either arrested, progressive, or regressive on the direct line of differential development of the given character in the ontogeny of the individual and the adults of asso- ciated species and genera. A variant in a species can, therefore, usually be compared either as a direct connection, or, if not that, as a parallelism with the typical condition in some other species of the group. It seems that Echini are a particularly good group in which to study questions of variation because here variations can usually be expressed in very definite terms of numerical or other equally positive characters. Structure of the Skeleton and Growth. The skeleton of a sea-urchin is made up of plates, spines, jaws, and other parts which are situated more or less deeply, and the skeleton is covered by living ciliated epithelial tissue. The plates and spines, while apparently external, are in reality included within living tissue. An exception to this covering of living tissue occurs in the primary spines of Cidaris, where, as Lang shows (1896, p. 389), a cortical layer is formed, and the integument dies away from the area covered by that tissue and persists only around the base of the spine. This may he com- pared with the velvet on deer's antlers, which, after the completion of growth, dies away, leav- ing the antler bare. The secondary spines of Cidaris, however, have no cortical layer, and are always covered by a ciliated integument, like a permanent velvet in antlers, and which is seen in the horns of the giraffe and okapi. This condition of the skeleton of Echini, of being covered by living tissue, is of fundamental importance to a proper understanding of the structure. As the parts are internal, they are capable of being added to or resorbed at any part through- out the life of the individual. This is true of all parts except the distal points of the teeth, which are truly external. The individual plate of the test grows by a constant addition to the exterior and resorption of the interior, which latter is composed of open lattice-like or trabecu- lar tissue. The growth of a sea-urchin plate may be aptly compared to the growth of the head of a femur or similar bone with its hard dense exterior and open trabecular internal structure. Sectioning a sea-urchin plate, we find no trace of its earlier shape or character within, any more than we find the traces of a young femur within in sectioning the femur of an adult dog. 52 ROBERT TRACY JACKSON ON ECHINI. The mineralogical structure of the sea-urchin plate, as seen in a section of a plate of Recent Strongylocentroius drobachiensis, is crystalline calcite, with tlie axis of the crystal perpendicular to the surface of the plate, as shown by extinguishment under the polariscope. The plate is made up of a crystal which is optically continuous. Whether one, or more than one crystal enters into the composition of a single plate is doubtful, l)ut it may jierhaps be compared to ice on a pond, in which also the crystalline structure is optically continuous and the crystalline axis is perpendicular to the surface. It may be noted that the same crystalline structure, with axis perpendicular to the surface, was observed in the plates of Synapta, Asterias, and the calyx plates of a young Antedon rosaccus. In the stem of Antedon, however, the crystalline structure is parallel to the longer axis of the stem, not perpendicular to the surface. While in the sea-urchin the corona grows by increase in size of the plates, often to a relatively great size, as in Cidaris, or the two plates in the second row of the posterior interambulacrum of Micraster, it also grows by the addition of new plates. In the corona, the new plates, both ^n the ambulacral and interambulacral areas, are always added dorsally, in immediate contact with an ocular plate. This holds good for Palaeozoic as well as later types; the only excep- tion appears to be the aberrant Pourtalesia jeffreysi Wyville Thomson, in which, according to Loven (1883), in adults, at least, oculars are apparently wanting. In Bothriocidaris archaica (Plate 1, fig. 2) it is seen that the ocular plates cover both the ambulacrum and interambu- lacrum entirely, and in all other Echini, as already stated, the oculars overhang the ambulacrum entirely and the interambulacra in part on either side, as in Melonechinus (Plate 50, fig. 6), and Lovenechinus (Plate 41, fig. 3). It is true that in many, j^erhaps most Echini, the interam- bulacral plates originate in contact with both the ocular and the genital plates in the angle between them, but they apparently never fail to come in contact with the oculars and they may not touch the genitals, as seen in Bothriocidaris archaica (Plate 1, fig. 2). In the posterior interambulacrum of Ananchytes (text-fig. 175) and in adult Phormosoma placenta (text-fig. 170) interambulacral plates originate against the oculars without touching a genital, and the same may often be seen in abnormal specimens as in area 4 of Arbacia (Plate 4, figs. 11, 12), and in the extraordinarily modified specimen of Strongylocentroius lividus (Plate G, fig. 5). It is strik- ing in the Palaeozoic genera with many columns of plates that the new interambulacral plates originate against the oculars, Lovenechinus, Plate 41, figs. 2, 3, as tliscussed more fully later. What part, if any, the ocular plays in the origination of new ])lates is miknown, but at this area they originate. Mr. A. Agassiz (1904, p. 80) notes that in sjiecimens of Lovenechinus (Oligoporus) rnissouriensis and Lepidechinus iinhricatus the young interambulacral plates originate against the ocular. The specimen of Lovenechinus to which he refers is that figured in my Plate 42, fig. G, and the Lepidechinus in my Plate 03, fig. 7, as L. tessellatus sp. nov. The periproct grows by increase in the size of plates and the addition of new plates quite independently of the corona. The peristome grows in area by increase in the size of basicoronal THE AMBULACRUM OF THE CORONA. 53 plates of the corona and in cases by resorption of the base of the corona. The plates of the peristome are various in origin. The primordial ambulacral plates of regular Echini are appar- ently in this area from their inception (Plate 3, fig. 7). Additional ambulacral plates are derived by flowing down from the corona (text-figs. 41-48, p. 80). All other peristomal plates, it is believed, are formed on that area and are not derived from the corona (text-fig. 57, p. 84). These matters will be considered in detail in their appropriate places. The Ambulacrum of the Corona. The ambulacrum is the most essential feature of a sea-urchin, and has a first importance in classification and morphology, on account of the varied structure that it presents. We can conceive of a sea-urchin without an interambulacnini, although such is not known, or wanting in almost any other skeletal parts, but it seems that an ambulacrum is an essential character of the class. The detailed structure of the ambulacrum in post-Palaeozoic types is for the most part fairly well known, but the Palaeozoic types have not been closely studied, and throw a great deal of light on the structure of this area in the group as a whole. The ambulacrum is essentially an area for bearing tube-feet, which are important as a means of locomotion in most Echini, or are modified more or less fully as organs of respiration in other Echini (clypeastroids and others). The ambulacrum is never composed of less than two vertical columns of plates, and when more than two, it always has an equal number on each side; therefore there is a distinct median suture with an equal area on each side of it. From this it follows that half an ambulacrum, barring exceptional variation, is as good as a whole one for studj^, which is important in fossils where one half is often all that is available. A partial exception to the rule of never less than two columns of ambulacral plates occurs in Palaeolropus josephinae Loven, as figured by Loven (1874, Plates 13, 32). In this exception, dorsally in all five areas the two columns drop out to a single column for a distance of three to seven plates from the oculars. This indicates that a sea-urchin might have a single column of ambulacral plates throughout at least the greater part of an area, but such is unknown. The width of the ambulacrum varies greatly. It is narrow usually, in types with low imiserial plates, Eucidaris (text-fig. 4), Palaeechinus (text-fig. 9) ; but it may be wide, as in the petaloid area of Clypeaster. The ambulacnmi is wider usually when the plates are compound, Strongy- locentrotus (text-fig. 5si), or when four colunms, Lovenechinus (Plate 45), or more columns, Melonechinus (Plate 57), are developed. It may be much wider than the interambulacrum, Lepidesthes colletti (Plate 70, fig. 1), Meekechinus (Plate 76, fig. 1), and about twice the width of the interambulacrum in Bothriocidaris (Plate 1, fig. 1). In the primitive type, Bothriocidaris archaica (Plate 1, fig. 1), the ambulacral plates are high, hexagonal, and there are therefore few tube-feet, about sixteen, to each area excluding 54 ROBERT TRACY JACKSON ON ECHINI. the two peristomal rows. In young Goniocidaris (Plate 2, figs. 1-3) the ambulacral plates are also high, hexagonal, and there are few tube-feet, about ten to each area. From this condi- tion, which I would call primitive, increase of tube-feet or increase of locomotive power, is attained on three independent lines of development. P'irst and simplest, by producing low plates with pore-pairs uniserial, so that many plates are contained in the vertical extent of the area, as in Archaeocidaris (Plate 9, fig. 6), Eucidaris (text-fig. 4), and Palaeechinus (Plate 30, fig. 3). A second method of getting tube-feet is by increasing the number of columns of ambu- lacral plates from two to a greater number, but always by even numbers and always by simple, never compound plates (text-figs. 10-21). The increase is from two to four, six, eight, ten, twelve, sixteen, or twenty columns of plates in an area, which is the most at present known. This method is known in the Palaeozoic Echini only, and is especially developed in the Palae- echinidae and Lepidesthidae. It is, however, taken up as a method in other types, as Echinocy- stites. The third method of attaining increase of tube-feet is by building compound plates, formed by the coalescence of originally simple plates, Centrechinus (text-figs. 92, 94). The Text-figs. 2-14. — Character of the ambulacrum in representative Echini; left half represented. The horizontal dotted hne is on the plane of the mid-zone. 2. Bolhriocidaris archaica sp. nov. Ordovician. From Plate 1, fig. 1. 3. Goniocidaris canaliculala A. Agassiz. Young. From Plate 2, fig. 2. 4. Eucidaris irihidoides (Lamarck). Bahamas. 5. Sirongyloccntrolus drobachiensis (O. F. Miiller). Young. From Plate 3, fig. 11. 5a. The same. Adult. York Harbor, Maine. 6. Micraster cor-anguineum (Lamarck). Cretaceous, England. 7. Echinaraclmius parma (Lamarck). East port, Maine. 8. Melalia pedoralis (Lamarck). Bahamas. Showing plates of two are;is. 9. Palaeechinus elegans M' Coy. Lower Carboniferous. From Plate 31, fig. 1. 10. Maccoya hurlingiomnsis (Meek and Worthen). Lower Carboniferous. P'rom Plate 33, fig. 2. 11. Lovenechinus missouricnsis (Jackson). Lower Carboniferous. From Plate 43, fig. 3. 12. Oligoporus danae Meek and Worthen. Lower Carboniferous. From Plate 50, fig. 7. 13. Mclonechinus Tmdtiponis (Norwood and Owen). Lower Carboniferous. From Plate 56, fig. 4. 14. Lepidesthvs colklli White, Lower Carboniferous. From Plate 70, fig. 3. THE AMBULACRUM OF THE CORONA. 55 component elements of a compound plate are frequently low, but the plate as a whole is usually high. This method occurs in the Centrechinoida and is seen in some of the Holectypina, but is unknown in the Palaeozoic. A good example is Strongylocentrohis drobachiensis (text-fig. 5a), which at the mid-zone is characterized by compound plates composed of five or six component parts, each of which bears a pair of pores. As a very rare aberrant variant, pores may be wanting in such plates. In three specimens of Strongylocentrotus drobachiensis, collected in Maine, the plates are compound, as usual, but in one area of each specimen the plates dorsally are somewhat distorted and have no ambulacral pores. It must be strongly urged that the full character of the ambulacrum in a given type is to be based on the plates at the mid-zone. Below this point, or ventrally, they may not have attained the full differential characters, and dorsal to the mid-zone, one gets into the area of locaUzed stages in development, where the young plates again have not taken on the full char- acter; or again, one may also dorsally reach an area of senescent stages where the full characters are dying out. To illustrate this important principle, in Maccoya burUngloncnsis (Plate 33, figs. 1, 2) the plates at the ventral border are all primaries and pore-pairs uniserial; at the mid- zone the plates are alternately primaries and occluded, and the pore-pairs biserial; dorsally, the young plates are again all primaries and pore-pairs uniserial. In Lovenechinus missourien- sis (Plate 42, figs. 1-4) the plates ventrally are all primaries, at the mid-zone demi- and occluded, and dorsally again primaries. In Melonechinus multiporus (Plate 56, figs. 2-7; Plate 57; text- fig. 245) ventrally, plates are demi- and occluded; at the mid-zone demi-, occluded, and many isolated; close to the ocular, primaries only. In all of these series, and many others as well, the same system exists of simpler conditions ventrally in the plates built when the animal was young, and also in the nascent plates dorsally ; and from there passing to the plates of the mid- zone, it is easily seen how the complex conditions there existent are built up. From these plates also one can arrive at genealogical relations, it is believed, of a very definite character (text-fig. 237, p. 231), as discussed later. Of the Centrechinoida, in the young Strongylocentrotus, Loven (1892) showed that the plates are simple with a single pore-pair (Plate 3, fig. 11). These plates in the adult are re- moved by resorption, but still the plates at the ventral border are unlike those of the mid-zone. At the dorsal border in Strongylocentrotus, as Loven (1874) showed, and as Duncan (1885) showed in a number of types, the plates are simple, not compound. Duncan says (1885, p. 421), "It is an interesting and highly suggestive truth that all the regular Echinoidea should have their most radially situated plates in the form of the simple primaries of the Cidaridae." Loven showed in Strongylocentrotus drdbachiensis that these simple plates are shoved down- ward and become fused to form the compound plates, characteristic of the species. At this region of young plates we find, therefore, as a localized stage, a simplicity like that of less spe- cialized forms of Echini, as (Uidaris. This character of dorsal simple plates is shown very well 56 ROBERT TRACY JACKSON ON ECHINI. in Centrechinus (text-fig. 94, p. 107), Astropyga (text-fig. 99), Echinus (text-fig. 115), and Phor- mosoma (text-fig. 170). Alexander Agassiz (1874, p. 642) said that the compound ainbulacral plates are formed by the splitting of the original plates, but he was clearly mistaken. Passing dorsally, we may pass from compound plates into an area of simple plates which have dropped all attempt at fusion, as seen in Hemi('idaris of the Jurassic (p. 17). Or, as A. Agassiz (1904, Plate 20, figs. 1-4) shows in Salenocidaris miliaris A. Ag., there is a single primary plate ven- trally succeeded by one compound plate, which is again succeeded by simple plates throughout the area, a case of extreme reversion, in which simplicity is the dominant character, and com- poundness or specialization is shown b.y only a single plate. This may be aptly compared to Baculites amongst the ammonoid cephalopods, in which its coiled (ammonite) character is reduced to a minute, almost microscopical basal portion, which is rarely preserved. What- ever the complexity of the ambulacrum in Palaeozoic or later types, it is striking that a simple condition obtains in the nascent plates of the placogenous zone. This all shows the fact of stages in development as evinced by the ambulacrum passing dorsally or ventrally toward the mid-zone, and the importance of differentiating zonal areas in the description of the ambulacrum. While the regular Echini maintain a pair of pores in each ambulacral plate,. or plate ele- menrt in the case of compound plates, the case is different in the irregular post-Palaeozoic Echini. In the clypeastroids below the petaloid area, the ambulacral pores soon drop out, as in Echi- narachnius parma (Plate 8, fig. 4), and the ambulacral plates for the most part at least are non- poriferous. It is true that there are ^'ery fine pores on the ventral side, as described by A. Agassiz (1874, p. 703), but these are so peculiar that they can fairly be distinguished from the usual pores of the ambulacral system. In spatangoids ventrally the pores may be want- ing, or reduced to a single pore, instead of a pair of pores to a plate, in numerous cases. As stated, ambulacral plates typically bear a pair of pores representing a single tube- foot, or the pair may be reduced to a single pore, as in some spatangoids. In spatangoids, however, we find a remarkable exception to the rule of a single tube-foot to a plate. In Colly- rites (Plate 3, fig. 15) and other types, as shown by Loven (1874), A. Agassiz (1904), and others, in the basicoronal row, the la, Ila, Illb, lYa, Yb have two pairs of pores, and this is the only case of two pore-pairs, indicating two feet to a simple plate, known in Echini. Loven thought it indicated a compound plate because of the two pairs of pores, or in cases two separate single pores, but always representing two tube-feet. The fact that no suture has been seen in these plates in any of this group of Echini militates against this view, and it seems best to consider this as a peculiar case of two feet to a. simple plate rather than a compound plate, the suture of which has not been observed. As discussed under consideration of these plates (pp. 69, 71), the ambulacral plates in the basicoronal row of spatangoids and clypeastroids may reason- ably be homologized with the first row of ambulacral plates found around the mouth in regular Echini and transferred to the basal row of the corona in the process of evolution of these spe- THK AMHI'LACRUM OF THE CORONA. 57 cialized forms. In spatangoids, tube-feet as locomotive organs are largely abandoned, and they have only or mainly a respiratory funrtion; here no compound plates are known. With a loss of the locomotive importance the ])lates below the petaloid areas, as in Micraster (text- fig. 6), are relatively high, even hexagonal, as in the embryo and the ancient Bothriocidaris. This primitive character of plates may exist in only one area. In Metalia pedoralis (text-fig. 8), for example, the plates at the mid-zone of the bivium and posterior pair of the trivium are low and wide, with pore-pairs horizontal. In the odd anterior ambulacrum, however, the plates are as high as wide, pore-pairs are superposed and in the middle of the plate, as in Bothrio- cidaris. In such plates we have often in the shape and height, also the superposed position of the pores in each pair, a condition which is a closer approach to the embryonic character and also to the character of Bothriocidaris than is seen at the mid-zone in the adults of any regular Echini excepting the ancient Bothriocidaris. The position and mutual relation of the ambulacral pores is of considerable interest. It appears to be typical of the primitive and great majority of adult Echini to have two pores for each tube-foot, and the pair of pores to be surrounded by a more or less strongly marked peri- podium. In many spatangoids the peripodium is absent. In all Palaeozoic genera there are two pores to every ambulacral plate and a peripodium is present apparently without exception (Plate 23, fig. 1). Of course it is such a slight eminence that it is very commonly worn off, but it has been found so widely that its frequent absence is attributed to conditions of erosion. In all Palaeozoic types and most modern Echini, the pores lie either in the middle of the ambu- lacral plate, which is rare, or nearer the next adjacent interambulacrum than the middle of the plate, which is the usual position. In no Palaeozoic type do they lie nearer the median suture of the ambulacrum than the middle of the ambulacral plate. In the demi-plates of Echinosoma hispidum (A. Agassiz, 1904, Plate 48) the pores lie nearer to the mid-suture than to the middle of the plates in which they occur. The same is true of occasional spatangoids, but is exceptional. In Bothriocidaris (Plate 1, fig. 1) the ambulacral pores of each pair lie in the middle of the plate, superposed doi-so-ventrally, and surrounded by an oval perijjodium, the axis of which coincides with the vertical axis of the area. In young Goniocidaris (Plate 2, fig. 2) the pores of each pair also lie superposed as in Bothriocidaris. From the superposed position, which may be called jirimitive, the position of the pores in all Palaeozoic types except Bothriocidaris and in most other regular Echini takes on a change in which there is a revolu- tion through more or less of an angle of 90 degrees from the perpendicular, the revolution being in such a plane that the upper pore of the two is revolved toward the next adjacent interam- bulacrum. The revolution is therefore to the left of the perpendicular in the left half ambu- lacrum and to the right of the perpendicular in the right half-ambulacrum. Only rarely in regular Echini does the revolution exceed 90 degrees from the vertical, and in no case that I have found does the revolution take place toward the middle of the ambulacrum, but in the 58 ROBERT TRACY JACKSON ON ECHINI. opposite direction. From thi.s it occurs that the pores of each pair occupy usually an inclined position, and the outermost pore of a pair is a little higher than the inner pore, Hyattechinus (Plate 23, fig. 1). It may be on a line with the inner pore, as in Melonechinus (Plate 61, fig. 8), but it is rarely lower than the inner pore in regular Echini. At the ventral border of the test the pores of each pair lie much more nearly vertical than at the mid-zone in Strongylocentrotus and Arbacia. In the latter the pores of the two pore-pairs adjacent to the median spheridium are actually superposed, as in Bothriocidaris. It is to be noted also that the pores on the peristomal ambulacral plates, as in Eucidaris (Plate 2, fig. 6), Phyllacanthus (Plate 2, fig. 18), Phormosoma (text-fig. 43, p. 80), and Strongylocentrotus (text-fig. 50), are nearly or quite vertically superposed as in the young and in the primitive Bothriocidaris. Also it may be shown that in very young, newly formed plates next the ocular, of Goniocidaris (Plate 2, fig. 4), the plates are high, not low, and the pores are nearly or quite superposed. This is in a measure like the young of Goniocidaris (Plate 2, fig. 2) or adult of Bothriocidaris. I have seen the same character in very young plates of Eucidaris tribuloides close to the ocular. In many clypeastroids, as Echinarachnius parma (Plate 8, fig. 4), the pores revolve through an angle of more than 90 degrees from the vertical, so that the outer pore of each pair lies lower than the inner pore. In many spatangoids the pores ventrally, and in most of the other ambula- cral plates, are superposed as in the embryo and Bothriocidaris, but in the petaloid areas they are nearly or quite horizontal. In Mctalia pedoralis (text-fig. 8) the plates in ambulacrum III are high, without petaloid expansion, and the pores are superposed in the middle of the plate; in the four other areas the plates are relatively lower, petaloid dorsally, and in these areas at least the pores are horizontal, not superposed, and not in the middle of the plates. From this it seems that the relative position of the pores represents a greater or less departure from the primitive, and that a more primitive or more specialized relative position of pores occurs at definite parts of the test in the same specimen. In adult Bothriocidaris, Palaeechinus (text-fig. 15), and in the young, as seen in Gonio- cidaris, Strongylocentrotus (Loven, 1892); also in the young, as shown at the ventral border of the test, in Maccoya (text-fig. 16; Plate 33, fig. 1) and Lovenechinus (Plate 42, fig. 1), we find that the pore-pairs in succeeding plates lie over one another in a continuous or uniserial system. This arrangement is certainly primitive. It is to be observed that, in this condition all tube-feet extend to the ground from one continuous line, and when the plates are low, that they are proximally closely crowded as in Cidaris. From this condition of uniserial pores and tube-feet, we pass in many independent series to types in which the pore-pairs are biserial, triserial, or polyserial in each half-ambulacrum. This is accomplished by alternate plates failing to reach the interambulacrum in whole or in part, thus producing a biserial arrangement, Maccoya (text-fig. 16; Plate 33, fig. 1) and Lovenechinus (text-fig. 18; Plate 42, fig. 2). In addition isolated plates may be introduced to make a triserial arrangement of pores, Melon- THE AMBULACRUM OF THE CORONA. 59 echinus springeri (text-fig. 19; Plate 52, fig. 2), Perischocidaris (Plate 67, fig. 1); or still more additional isolated plates may be introduced to make many series of pores in each half-ambula- crum, Melonechinus muUiporus (text-fig. 20; Plate 56, fig. 4), Lepidesthes colletti (text-fig. 21; Plate 70, fig. 1), and Meekechinus (Plate 76, fig. 1). In the Centrechinoida the pores of the 3[o 2{^" 1 (CD 6KZ 7 \ 4(^ 5 > 2^ 3 > 1(0 ) Text-figs. 15-21. — Ambulacra of Palaeozoic Echini showing development of pore series by a drawing-out process of plate movement. 15. Palaeechinus elegans M'Coy. Lower Carboniferous. Adapted from Plate 31, fig. 1. All plates reach the interambulacrum . 16. Maccoya huiiiiiglotiensis (Meek andWoTthen). Lower Carboniferous. Adapted from Plate 33, fig. 1. Venlrally all plates reach the interambulacrum; further up, only alternate plates. 17. Lovenechinus sepHcs sp. nov. Lower Carboniferous. Adapted from Plate 45, fig. 2. Ventrally all plates reach the interambulacrum; further up, every alternate plate. At the mid-zone the plates are as in te.xt-fig. 18. 18. Loi'etiechinus missouriensis (Jackson). Lower Carboniferous. Adapted from Plate 42, fig. 2. Every second plate reaches the interambulacrum. 19. Melonechinus springeri sp. nov. Lower Carboniferous. Adapted from Plate 52, fig. 2. Every third plate reaches the interambulacrum. 20. Melonecldnus mulliporus (Norwood and Owen). Lower Carboniferous. Adapted from Plate 56, fig. 4. Every fifth plate reaches the interambulacrum. 21. Lejddesthes collclti Wliite. Lower Carboniferous. Adapted from Plate 70, fig. 3. Every eighth plate reaches the interambulacrum. 60 ROBERT TRACY JACKSON ON ECHINI. compound plates tend strongly to fall into arcs in such fashion that succeeding pore-pairs fall in diiTerent vertical series, as seen well in Strongylocentroius franciscanus. In Diplocidaris and Tetracidaris the pore-pairs, though in primary plates, are alternately pulled out of line, so that succeeding tube-feet would not lie quite over one another. This passage from the primitive condition of monoserial pore-pairs to the Ijiserial or poly- serial, which is attained by the three methods of more than two columns of simple plates (Palaeozoic), compound plates with pores tending to arcs (Centrechinoida), or primaries with pore-pairs biserial (certain Cidaridae), is all in accordance with the mechanical principle of each tube-foot attaining fullest play by being out of line with its fellows in so far as is possible. The arraiigement reminds one of the phyllotactic arrangement of leaves in plants, where e?tch leaf is not succeeded by another in a vertical line imtil one or more ha\'e been added which are off that vertical line, but in a succession of lines of their own series. In types, as spatangoids, in which below the petals the plates are relatively high and tube- feet therefore not crowded, the pore-pairs fall nearly in vertical uniserial lines, as in the J'oung and primitive forms. This is aided probably by the fact that the ambulacral feet are for the most part modified as respiratory rather than locomotive organs. Ambulacral plates on their proximal side differ from the same plates on their distal side, and this difference, as far as observed, is usually in the line of greater simplicity and relative primitiveness of structure. In Melonechinus mulliporus (Plate 56, figs. 4, 5) and M. gignn- teus (Plate 61, figs. 5-9), the ambulacral pore-pairs distally are eccentric and lie nearest to the interambulacral suture, but proximally, as shown, the pore-pairs of isolated plates lie in the middle of each plate, a primitive character. In general the pores proximally are nearer the middle of the plate than on the distal side. In Maccoya intennedia (Plate 34, fig. 2) the plates alternately reach and are cut off from the interambulacral suture and pore-pairs are biserial. All this is as seen from the outer or distal side. When seen from the inner or proximal side (Plate 34, fig. 3), all the plates at \\w mid-zone ci'oss the half-area instead of alternate plates being cut off from the interambulacral suture. iVlso the pore-pairs are uni- serial instead of biserial, as they are on the outer side of the ^•erv same plates, so that in all these characters they are primitive like the young and the next lower genus Palaeechinus (Plate 31, fig. 1). A similar condition is seen in distal and proximal sides of the plates in Maccoya burlinglonensis (Plate 33, figs. 4, 5). In Lovenechinus missouricnsis the outer demi-plates, as seen distally (Plate 43, fig. 3) are verj^ narrow, the inner occluded plates are relatively wide and the pore-pairs of both lie very near the interambulacral suture. In a proximal view of the same plates (Plate 43, fig. 4) the demi- and occluded plates are of about the same width, and the pore-pairs of both lie near the middle line of the half-ambulacrum. Also proximally the plate that lies opposite the horizontal suture lines of the adradial plates is spread out in a fan-shaped manner, whereas on the distal side the fan-shape is nearly or quite wanting. THE AMBULACRUM OF THE CORONA. 61 V A similar condition of fan-shaped plates proximally, which are not so shaped distally, is seen in Melonechinus multiporus (Plate 5G, figs. 4, 5) and M. giganteus (Plate 61, figs. 5-9). Comparable differences between the interior and exterior sides of the same plates are seen in interambulacral, ocular, and genital plates, as described in the consideration of these parts (pp. 75, 96, 172). All this shows that in the study of specimens and the description of species, one must bear in mind whether the specimen represents the internal or external characters of the test. Another feature noticed on the inside of ambulacral plates is elevated nodose or spinose projections that occur in some Echini. These were first seen in the fossil Hyattechinus (Plate 24, fig. 6), where low, knob-like or spinose elevations exist between th(> inner pore and the middle of the area. Alexander Agassiz (1904, p. 31) has described spines extending into the body from the inner face of the peristomal ambulacral jilates of Porocidaris cohosi, but I believe they have not been noticed before in coronal ambulacral ])lates. In Phyllacanthus (Plate 3, fig. 12) near to the ventral border of the test, there are small spines between the inner pores and the middle of the area; these seem to be the equivalent of those occurring in the Palaeozoic Hyattechinus. Close to the peristomal border these spines increase in height, arch over and then fuse in a continuous ridge (text-fig. 224, p. 193). Passing dorsally, in Phyllacanthus a second series of spines occurs, one over each inner pore, and above the ventral area these alone exist. In Eucidaris tribuloides I find a similar condition to that of Phyllacanthus, except that there are commonly three spinules over the inner pore (Plate 3, fig. 13). tSuch spinose growths are most delicate and are destroyed by a touch so that they can only be seen in a test cleaned without any brushing. They have not been noticed in any of the Centrechinoida. Closely associated with the ambulacrum are the spheridia described by Loven (1874), which, as he showed, are characteristic of all modern Echini except the Cidaridae. If they had existed in pits, as is so often the case, we might expect to find them in the Palaeozoic. The fact of their absence in the primitive Cidaridae is an argument for the assumption that they were also probably absent in Palaeozoic genera. Certainly they are not known. Pedicellariae have recently come into prominence through the critical studies of Mortensen and others. It is of interest to note that tridentate pedicellariae occurred in the Palaeozoic, as shown in Meekechinus elegans gen. et sp. nov. (Plate 76, figs. 8, 9). Except as shown in the Jurassic Pelanechinus by Clroom (1887) I believe these are the first found fossil, and they will probably always remain rarities. Special respiratory organs are also closely associated with the ambulacra. Charles Stewart (1879) first described internal branchiae in Eucidaris iribuloides which lie dorsal to the lantern and extend over the ambulacral areas radially from beneath the compasses and between these and the braces. The same structures were described by Ludwig (1880) and by Prouho (1887) in Cidaris papiUata. Prouho gave them the name of Stewart's organs, a convenient distinction from the quite dilTerent external gills. I have seen them in dissections of Cidaris affinis, 62 ROBERT TRACY JACKSON ON ECHINI. Eucidaris tribuloides and thouarsii. Stewart's organs are described in Asthenosoma by P. and F. Sarasin (1888), and in several echinothuriids by A. Agassiz and Clark (1909). External gills are developed as five pairs of branched appendages which are outgrowths of the oral integu- ment (text-fig. 55, p. 83). They are characteristic of the Centrechinoida and by inference the fos- sil Holectypina. Their presence is marked by the indenting of the interambulacral basicoronal plates so that they are recognizable in fossils. These peristomal gills, as I would call them for distinctiveness, are interambulacral in position in contradistinction to Stewart's organs, which are ambulacral or radial in position. In clypeastroids and spatangoids, as well as par- tially in some of the Centrechinoida (Arbacia), the function of respiration is maintained by the dorsal ambulacral tentacles, which have lost their function as locomotive organs. These I would call in distinction ambulacral gills. We do not and probably cannot know definitely what respiratory organs existed in Palaeozoic Echini. Branchial slits for peristomal gills I have found no evidence of, though carefully looked for. There is no evidence of specialization of tube-feet as ambulacral gills, though they may have performed that function in part. As the primitive Cidaridae have the Stewart's organs so well developed, it is not unlikely that these organs were the respiratory organs of all Echini in Palaeozoic times. The Interambulacrtjm. The interambulacrum in Echini functions chiefly as a space filler and a bearer of spines and pedicellariae. The spines serve for protection and more or less in locomotion, and pedi- cellariae as grasping, cleansing, and protective organs. In spite of this secondary physiological importance, the interambulacrum forms a large part of the test of the sea-urchin in most types, and is of very great interest, especially in Palaeozoic genera. The interambulacral plates originate in direct contact with the ocular plates and quite independently of the genitals. The young last added plates may always be found in direct contact with the oculars in Palaeozoic and later Echini, and their independence of the genitals is proved in cases where no genital reaches the interambulacrum, as in Bolhriocidaris archaica (Plate 1, fig. 2), the posterior area in spatangoids (text-figs. 174, 175, p. 149), the aberrant Arbacias (Plate 4, figs. 11, 12), and Tripneustes (Plate 6, fig. 4). As any given interambulacrum comes in contact with two oculars, one on either side, and as at the base of these two oculars new interambulacral plates originate (Melonechinus, Plate 56, fig. 6, and numerous other figures), it seems that an interambulacrum may theoretically be considered as composed of two halves, one half-interambulacrum being associated with the ocular and ambulacrum on one side, and the other half being associated with the ocular and ambulacrum on the other side. In other words, the corona may be conceived as made up of five areas, each surmounted by an ocular plate, an ambulacrum and adjacent half-interambulacrum on either side making up each THE INTERAMBULACRUM. 63 of these five parts. Such an ideal area composed of plates 'associated with a single ocular is shown in Palaeechinus and Eucidaris (text-figs. 217, 218, p. 191). Support for this view is found in the fact that the perignathic muscles of the lantern, which are nearly or quite ambulacral in position, are inserted wholly on the base of the associated two half-interambulacra, as in Eucidaris (text-fig. 218); or if not, they are inserted on two half-interambulacra and auricles of the ambulacrum itself, as in Centrechinus (text-fig. 219, p. 191). In the Echinothuriidae where oculars and genitals are separated by an interspace, Mr. Agassiz (1883, p. 32; 1904, p. 96), says that the interambulacral plates are derived from the periproct, as the two areas are there in contact. Dr. Mortensen (1903, p. 175) opposes this view. Evidently this contact is a mere coincidence of structure and has no bearing on the origin of plates. In the echinothuriids which I have seen, the young interambulacral plates are in contact with the oculars as usual (text-fig. 170, p. 149). Wliile in regular Echini outside the Echinothuriidae the oculars and genitals typically form a closed ring, yet occasionally in variants the ring may be open so that the periproct reaches the interambulacrum, as in Strongylocentrotus (Plate 5, fig. 15; Plate 6, fig. 5). The full differential characters of the interambulacrum as of the ambulacrum are expressed at the mid-zone of the adult. Here are usually found the full number of columns of plates characteristic of the species, also the typical tubercles, spines, imbrication, or other characters which go to make up the specific description. The ventral border in the basicoronal zone represents the earliest formed plates and the youth of the individual, as far as it can be gathered from the study of an adult specimen, though the actually first formed plates may have been resorbed in development. Passing dorsally, with later added plates, new characters may come in until we get the full differential features developed at or about the mid-zone. Dorsal to the mid-zone we pass into the area of young last formed plates which have not yet acquired the full characters. Or again dorsally, we may find senescent features in the loss of columns of plates. Passing from the basicoronal row dorsally, we find in most Palaeozoic types, and many post-Palaeozoic as well, stages in development strongly marked, which stages can be correlated with the adult condition of simpler genera or simpler species within the genus. The interambulacrum in Echini has from one to fourteen vertical columns of plates in each of the five areas, which represents the least and greatest number known at present. There are intermediate grades representing every step between this least and greatest specialization of the area, and it is a matter of great interest to follow the progressive series as represented by stages in development, and by adult types, to see how the progressive differential structure is built up. As the plates of the ventral border are the oldest or first formed of any plates seen in an individual specimen, and as the later added plates succeed one another as we pass dorsally, it might be thought that we could read stages in development as expressed by rows and columns of plates with ease and certainty, and such can be done in many types, as in 64 ROBERT TRACY JACKSON ON ECHINI. Hyatlechinus beecheri (Plate 26) aiid Perischodomus biserialis (Plate 64, fig. 2). Complications may come in, however, especially resorption of the base of the corona by encroachment of the peristome cutting off part of the ventral plates, and also rarely resorption within the corona, as exceptionally in Arachnoides, or differential growth of associated plates, which may separate plates originally in contact (Echinarachnius). There may be few plates in a given interambulacrum, as in Bothriocidaris (Plate 1, fig. 1) or Cidaris, or a very large number, as in Hyattechinus beecheri (Plate 26), and many other species. There is every evidence that these plates all originated as separate plates and never by division of previously formed plates. Mr. Agassiz (1881, p. 95; 1904, p. 104) saj's that in Phormosoma, interambulacral plates are formed by the splitting of earlier continuous plates. Mortensen (1904, p. 25) suggests that these planes of division may be due to fracture in the handling of the specimens which have excessively thin plates, and my limited observations of Echinothuriidae accord with this view. If these are natural suture lines in Phormosoma, the numerous plates thus formed cannot bo homologized with the numerous columns of plates in Palaeozoic types to which Mr. Agassiz compared them, for the latter originate independently, not by the splitting of previously continuous plates. While fusion into compound plates is seen in the ambulacrum of the Centrechinoida, such fusion is unknown in the interambulacrum in all Echini. Loven (1874, p. 51) assumed, it is true, fusion of certain plates around the base of the corona in spatangoids. No sutures exist, and it seems that single plates at this area in these and other types are not cases of fusion, but of failure to develop more than one plate in the row to which they belong. In the important ancient type Bothriocidaris, as best shown in B. archaica (Plate 1, fig. 1), there is a single column of interambulacral plates in each area extending from just above the second row of ambulacral plates of the peristome to the apical disc, where in this species they are completely covered by the large ocular plates and do not reach the genitals. The full details of this and other cases given are considered under the description of the species. This is the simplest and, I think, the most primitive condition known in any adult echinoid. In my paper on Echini (1896, p. 233) I considered Bothriocidaris as representing the archaic radical as regards interambulacral structure from which the interambulacra in all other Echini could be derived. Further evidence and stutly confirm this view. Mr. A. Agassiz (1904, p. SO) says, "The existence of an interambulacral zone composed of a single row of plates [in Bothriocidaris] does not give us any clue to the mode of formation of the Palaechinid type of interambulacra with its manifold rows of plates." I cannot agree with this view, as it seems that the single primordial plate found so universally in j'^oung Echini, or in adults when resorption has not removed it, may fairly be considered as representing in development a stage consisting of a single column which in all but Bothriocidaris is succeeded by additional columns during development, as shown in my first paper and in numerous cases THE INTERAMBULACRUM. 65 in this paper. Dr. Mortensen (1903) describe.s a very young Hypsiechinus (my Plate 3, fig. 6), in which the interambulacrum consists of but a single plate in each of the five areas. In young Echinus (Plate 3, fig. 5) Bury (1895) figures a similar single plate, and Loven (1874, Plate 17, fig. 49) figures a young echinoid in which there is only a single interambulacral plate in each area. This single plate, it seems, represents a single-column stage consisting of this one plate, and may fairly be homologized with the plate at the ventral border of the interambulacrum of Bothriocidaris. Loven (1892) showed that in young Goniocidaris (my Plate 2, fig. 1), also in Strongylo- centrotus (Plate 3, fig. 11), and Echinus, at the ventral border of the interambulacrum there is a single plate which lies dorsal to the continuous row of primordial ambulacral plates. This plate is evidently the single first formed plate, and is homologized with the ventral plate of the interambulacrum in Bothriocidaris. In the young of the Recent Echini mentioned, the first plate is succeeded in the second row by two plates introducing the character of a second column, which is the next stage in differential development of the interambulacrum. A similar single first plate succeeded by two plates in the second row is shown by A. Agassiz (1904) in young Salenia (his Plate 21, fig. 1), young Phormosoma (my Plate 3, fig. 10), and in young Arbacia (his Plate 54, fig. 2). In later life, in regular Echini, as shown by Loven, this single initial plate is usually resorbed by the encroachment of the actinostome, so that in the adult there are two plates on the peristomal border, as in Cidaris and most regular Recent Echini. Arbacia (text-fig. 227, p. 193) and Phormosoma (text-fig. 43, p. 80) are exceptions, for, as Mr. Agassiz showed (1904, p. 54, Plate 41), the single plate is retained at the ventral border of the adult in these genera. As Loven (1874) showed in the clypeastroids and spatangoids, the plate is retained at the ventral border of the corona in most cases, as the corona is not resorbed by the encroachment of the peristome. There are certain exceptions to this, as later discussed. Turning to Palaeozoic types, we find in Perischodomus (text-fig. 30, p. 70; Plate 64, fig. 2) that there is a single plate ventrally succeeded by two plates in the second row, as in young Cidaris, etc., but in the third row there are three plates, which introduces another stage in differential development, structurally an advance on that seen in Cidaris and all modern Echini. In the next row of Perischodomus there are four plates, and later a fifth column is introduced. In this type we see that a form with five columns of plates in the adult is built up by a series of stages starting with a single column represented by a single plate ventrally, and the succeeding stages are represented by additional rows or zones of growth progressively introducing new columns or new characters in differential development. In Hyattechinus beecheri (Plate 26) we find ventrally a single plate representing a single ' In his text Dr. Mortensen (1903, p. 89) calls this plate a genital, but he wrote me that this was an error, and in the second jjart of his Ingolf Eehinoida (1907, p. 172), corrects the error, saying that these plates are the first interambulaerals and the genitals are discernible on the abactinal side. 66 ROBERT TRACY JACKSON ON ECHINI. column of plates in each of the five areas. In the second row there are two plates, in the third three, in the fourth four, in the fifth five, and so on. There is not always a new column added in each new row or zone of growth in this species, but this is very nearly the case up to the maximum number of columns added, which is eleven. Dorsally the adradial columns 1 and 2 drop out, so that columns 3 and 4 come against the ambulacrals in further dorsal growth. The same series of stages of progressive development marked by adding columns, and regressive senescent stages marked by the dropping out of columns of interambulacral plates are shown in Hyattechinus rarispinus (Plate 23, figs. 1, 3) and H. peniagonus (Plate 25, figs. 1, 3). In this last species in senescence, several columns are dropped, so that close to the apical region there are ten columns instead of the fourteen characteristic of its full development near the mid-zone. H. pentagonus is also especially interesting, as it attains fourteen columns of plates in the adult in each area, the maximum specialization in this line of differential development of all known Echini. Yet as resorption has not removed any plates, we can trace the development from the simple ventral or young condition, represented by a single plate on the peristomal border, to the complex adult and senescent old age without a single gap in the series of stages in develop- ment. In other Palaeozoic genera a similar progressive individual development may be traced in which the single ventral plate is retained in the adult, as in Pholidechinus (Plate 28, fig. 1), Pholidocidaris (Plate 73, fig. 6), Lepidocentrus (Plate 20, fig. 7), and Lepidesthes (Plate 68, fig. 3). The above represent Palaeozoic genera in which there is no ventral resorption of the corona, and therefore all the interambulacral plates, built during the life of the individual, are retained and can be seen in a well preserved adult individual. Such successive stages in the development of an interambulacrum can be fairly compared to studying the development of a cephalopod or other mollusc, as has been done so successfully by Hyatt and others, by tracing the characters from the umbo or original apex of the shell by a series of steps or stages to the completed adult or old-age individual, all worked out in a single specimen. There is the great difference, however, that in the mollusc the shell is external, in the sea-urchin internal, so that in the latter the plates have grown since first formed and may have suffered changes, but from their presence stages can be read. Resorption, however, comes in in Palaeozoic as well as in post-Palaeozoic types. Studying genera representing the Palaeechinidae, we find that there are two plates at the ventral border of the interambulacra in all cases where the ventral part of the corona is complete, as seen in Palaeechinus (Plate 30, fig. 3), Maccoya (Plate 33, fig. 1), Lovenechinus (Plate 43, fig. 1), Melonechinus (text-fig. 25; Plate 50, fig. 2; Plate 57, fig. 1), and others. This character is explained by the fact that the single initial ventral plate, found so universally in young modern Echini, and retained ventrally in numerous fossil and living genera, has been resorbed in the advancement of the peristome, as in Cidaris and most modern regular Echini. Miss Klem THE INTERAMBULACRUM. 67 (1904, p. 3) criticizes this view and claims that there are more than two plates when tlie ventral area is complete. I can only say that Miss Klem is mistaken. I have never seen an exception. Two of the specimens here figured, showing this character of two plates ventrally, are from the Hambach Collection which Miss Klem studied and herself figured, but it is fair to state that they have been freed somewhat from matrix since she studied them. The specimens referred to are Lovenechinus septies (Plate 45, fig. 1 = Miss Klem's Plate 2, figs. 5a-5d) and Melonechinus muUiporus (Plate 57, fig. 1 = Miss Klem's Plate 3, figs. 6a-6d). In Palaeechinus quadriserialis (Plate 30, fig. 3) there are two plates in the first row or zone, three in the second, four in the third (excepting in one area, C), and above the introduction of the fourth column no more columns are added, but all are continued to the apical disc. Loven (1874, p. 39) made the acute observation that in Palaeozoic genera with multiple columns of plates, the adambulacral plates alone are continued from the peristome to the apical disc. This observation holds true of most of the family of the Palaeechinidae, as in Palaeechinus (Plate 30, fig. 3) and Lovenechinus (Plate 41, fig. 1). The statement, however, needs modification. In those types where there is a single plate at the ventral border, Perischodomus (Plate 64, fig. 2), obviously two columns do not reach the peristome. Also in senescence (Melonechinus indianensis, Plate 53, fig. 1), usually the two adambulacral columns drop out before reaching the apical disc; or in special- ized types they may drop out very early, as in Hyattechinus beecheri (Plate 26) or H. pen- iagonus (Plate 25, fig. 3). In the family of the Palaeechinidae the species have as a range from three to eleven columns of interambulacral plates. In the several species the early development is as in Palaeechinus quadriserialis, but in later growth additional differential characters are usually taken on. In a young Lovenechinus missouriensis (Plate 40) a fifth column comes in soon after the fourth, and in this specimen, which is very young and the only really young Palaeozoic echinoid yet known, the fifth column originates near the mid-zone. In an older individual (Plate 41, fig. 1) where many more rows of plates have been added dorsally, we find that the introduction of the fifth column has been relatively shoved ventrally by the later added plates. In addition, a sixth column is represented by one or two plates in three areas, E, G, I, but in two areas, A, C, no sixth occurs. This occurrence of a column represented by only one or two plates is very exceptional in Echini and suggests a disappearing part. In many specimens of the species it is entirely absent. In Lovenechinus septies the method of introduction of columns is well shown (Plate 45, fig. 1), as it is one of the best preserved specimens I have seen. In three areas the two plates are present in the basicoronal row, restored in the other two; from this point up or dorsally in the several areas the new columns come in quite regularly. In the very choice specimen of Melonechinus muUiporus (Plate 57, fig. 1; text-figs. 245, 246) there are two plates ventrally in areas G, I, one of the two in areas A and C, and they are restored in areas where wanting, as indicated by dotted lines. Above this zone the introduction of columns is shown without the absence of a single plate in all five interambulacral areas. 68 ROBERT TRACY JACKSON ON ECHINI. In Melonechinus muUiporus there are eight or nine, rarely seven, columns of plates in each area. In M. vanderbilti there are nine columns of plates, and in M. giganteus (Plate 59, fig. 14) there are eleven. The columns are all added in a perfectly definite method and are in this feature further developments of the system seen in Palaeechinus quadriserialis. The progressive addition of columns as we pass dorsally from the single column ventrally to the two to fourteen columns dorsally, I believe, represents distinctly stages in development through which the animal passes in its growth, the progressive stages being comparable to the conditions seen in the adult of simpler genera or simpler species allied to the type under con- sideration. The method of introduction of interambulacral columns was shown in detail by Jackson and Jaggar (1896) and Jackson (1896), but will be briefly stated here. In the Palaeechinidae and most Palaeozoic Echini, excepting those that have strongly imbricate plates, the plates of the interambulacrum are typically hexagonal excepting the adambulacral columns which are nearly pentagonal but crenulate or rounded on the adradial suture. WTiile the plates are typically hexagonal, this symmetry is modified where new columns are introduced. The initial plates of columns above the third are typically pentagonal with an apex of the pentagon pointing ventrally, and an adjacent ventral plate on the left or right is heptagonal, the added side com- pensating for the one side short in the initial pentagon. An exception to this rule of pentagonal initial plates of columns is seen in the initial plate of the third column, which is typically hex- agonal and is immediately succeeded by the initial plate of column 4, which is a pentagon, as seen in Melonechinus muUiporus (Plate 57) and many other species. If by rare exception the initial plate of column 3 is not immediately followed by the initial plate of column 4, as in area C of Lovenechinus septies (Plate 45, fig. 1), then the initial plate of column 3 is also pen- tagonal. When odd-numbered columns, as columns 3, 5, 7, 9, 11, etc., are introduced, they are typically introduced in the middle of the area with an equal number of columns on each side of them, and also typically the heptagonal associated plate lies on the right ventral border of the initial pentagon. When even-numbered columns are added, as columns 4, 6, 8, 10, etc., they are typically added to the right of the center with one more column on the left than on the right. While this is the rule in the large majority of cases, there are frequent exceptions in which even-numbered columns are added on the left instead of the right of the center. As a concurrent character, the associated heptagonal plate in even-numbered columns typically lies to the left of the initial pentagon. Why this systematic position of columns and associated heptagonal plates should come in with such regularity is not obvious, but it is a very constant character, as shown by Jackson and Jaggar {1896, p. 163) who worked out the percentages of normal cases and variations. In llie dorsal portion of the interambulacrum the plates are nearly or quite rhombic in outline and gradually take on the typical hexagonal outline, as they are pu.shed ventrally by later intercalated plates and come into mechanical contact with adjacent plates of their several associated columns (Plate 59, fig. 14). CHARACTERS OF BASICORONAL PLATES. 69 Dorsally each interambulacrum comes in contact with a genital and two adjacent ocular plates, except in Bolhriocidaris archaica (Plate 1, fig. 2) and in part in B. pahleni (Plate 1, fig. 6), and in the posterior interambulacrum of many spatangoids (Micraster), where the interambula- crum abuts against two ocular plates only (text-figs. 174, 175, p. 149). The interambulacrum also comes against oculars only in the remarkable variations in Arbacia shown in Plate 4, figs. 11, 12. The new interambulacral plates in Echini are formed against the ocular plates on either side of the area, never in the middle against the genital. The relation of the interambulacrum to the ambulacrum in development is of much interest, and it seems may be expressed in terms of acceleration of development. In Bothriocidaris (Plate 1, fig. 1) two continuous rows of ambulacral plates surround the mouth, forming the peristome of that type, before reaching the initial plate of the interambulacrum in the third aboral row or zone. This character in Bothriocidaris is discussed in detail under consideration of that type. In young Goniocidaris (Plate 2, fig. 1) and in young Strongylocentrotus (Plate 3, fig. 11), young Salenia (A. Agassiz, 1904, Plate 21, fig. 1), and young Phormosoraa (Plate 3, fig. 9), a single row of continuous ambulacral plates surrounds the mouth forming the peristome at that age, but the initial single plate of the interambulacrum appears in the second row which is the base of the corona, instead of the third row as in Bothriocidaris. In clypeastroids and spatangoids (Collyrites, Plate 3, fig. 15) the initial single plate of the interambulacrum appears intercalated between the plates of the primordial ambulacral row, which, therefore, are not continuous as in regular Echini, but discontinuous, and, moreover, do not form part of the peristome, but form with the primordial interambulacral plates the basicoronal row of plates of the corona proper. Characters of Basicoronal Plates. Basicoronal plates are the oldest or first formed plates existent in any given corona, and unless plates have been removed by resorption, or by flowing down on to the peristome, as in the ambulacrals of cidarids, they are the actual plates formed at this area in the young animal. Therefore, as elsewhere discussed, they often present simpler characters than plates dorsal to them, and thus show stages in development. These plates on the proximal side give rise to the attachment of part of the muscles of the Aristotle's lantern, either directly or through the elevated processes of the perignathic girdle, which is later discussed. There are certain types of structure in basicoronal plates which are comparatively constant in large groups, so that they are of more than specific or generic value. The characters of basicoronal interambulacral plates are the more striking and may be stated in brief. Where no plates have been removed by resorption, there is a single plate at the ventral border of the interambulacrum. The primitive type of this character is Bothrio- 70 ROBERT TRACY JACKSON ON ECHINI. cidaris (text-fig. 22), which continues to build a single column. This same character of a single plate ventrally, but succeeded by two plates in the second row, is characteristic of the young of all modern regular Echini, as shown by Loven (1892) (Goniocidaris, text-fig. 23). In the adult of most regular Echini the single plate and probably more have been resorbed by the advance of the peristome (Eucidaris, text-fig. 24). In the Palaeechinidae with many columns of plates, apparently only one plate has been resorbed, when we find two plates in the basicoronal 27 28 29 30 31 Text-figs. 22-31. — Characters of the base of the interambulacrum in representative Echini. 22. Bolhriocidaris archaica sp. nov. Ordovician. From Plate 1, fig. 1. 23. Goniocidaris canaliculala A. Agassiz. Young. From Plate 2, figs. 1, 2. , 24. Eucidaris tribuloidcs (Lamarck). Bahamas. 25. Melonechinus muUiporus (Norwood and Owen). Lower Carboniferous. From Plate 57. 26. Archaeocidaris wortheni Hall. Lower Carboniferous. From Plate 9, fig. 6. 27. Plexechinus cinclus A. Agassiz. Adapted from A. Agassiz, 1904, Plate 58, fig. 1. 28. Echinocyamus -pusiUxis (Miiller). Adapted from Lov6n, 1874, Plate 44. 29. Rotula dentata (Lamarck). Adapted from Lovfn, 1874, Plate 46. 30. Pcrischodomus hiscrialis M'Coy. Lower Carboniferous. From Plate 64, fig. 2. 31. Tiarcchinus princeps (Laube). Triassic. Adapted from Loven, 1883, Plate 13, figs. 152, 154. In figures 22, 23, 27-31 the primordial interambulacral plate is in the basicoronal row; in 24-26, it, with or without additional plates, has been resorbed. row (Melonechinus, text-fig. 25), or in the Archaeocidaridae, several rows of plates may have been resorbed, and we find four plates in the basicoronal row (Archaeocidaris, text-fig. 26). In modern Echini where there has been no resorption, the single primordial plate is retained in the adult. This character, as shown by Loven (1874) and others, occurs in most clypeas- CHARACTERS OF BASICORONAL PLATES. 71 troids and spatangoids (text-figs. 27-29). In Plexechinus (text-fig. 27) the .single plate is followed by another single plate, and these by a second column. This is a very rare exception, as the typical character is for the initial primordial plate to be immediately succeeded by two plates in all Echini except Bothriocidaris. In Rotula (text-fig. 29) the primordial interam- bulacral plate is separated from the next two of its area by the lateral introvenient ambulacral plates. As shown by Loven, plates originally in contact in the young may be thus separated during growth. A few modern regular Echini retain the primordial plate in the adult (Phor- mosoma, text-fig. 43, p. 80; Arbacia, text-fig. 227, p. 193). In Palaeozoic Echini with imbricate plates and many interambulacral columns, there is no resorption, and therefore the primordial interambulacral plate is retained in the adult, as in Perischodomus (text-fig. 30), Hyattechinus (Plate 25, fig. 1), Lepidocentrus, Lepidesthes, and Pholidocidaris. The primordial plate in all these is succeeded by two plates, then three, then four, etc. (text-fig. 30). In the remarkable Tiarechinus from the Trias, Loven (1883) showed that the primordial interambulacral plate is succeeded by three plates (text-fig. 31), and then no more plates are built, a unique condition. Considering the general characters of the basicoronal row, with special reference to the ambulacral plates, we find the following types of arrangement. In Bothriocidaris the basi- coronal row consists of two high hexagonal ambulacral plates with pores superposed in each ambulacral area and one interambulacral plate in each interambulacral area (Plate 1, fig. 1). This same character is seen in young cidarids (Plate 2, fig. 2) (Loven, 1892; A. Agassiz, 1904), young Strongylocentrotus (Plate 3, fig. 11), and Echinus (Loven, 1892), young Salenia, Arbacia, and Phormosoma (A. Agassiz, 1904). It is, I think, fair to call this a primitive character, and it represents what I (1896, p. 235) described as the protechinus stage. The protechinus stage is comparable in other groups of animals to the protoconch of cephalous Mollusca, what I (1890) described as the prodissoconch of Pelecypoda, and to Beecher's (1901) protegulum of Brachio- poda and protaspis of Trilobita. All are referable to what I termed (1890, p. 289) the phylenibryonic stage in development, a stage in which the differential characters of the class are established in ontogeny. A second type of basicoronal plates, in which there are two ambulacral plates alternating with one interambulacral plate in each area, is seen in types where the ambulacral plates have in part flowed down on to the peristome, so that the plates originally there are gone, and yet the primordial interambulacral plate has retained its original place. Such is seen in the Palaeo- zoic Hyattechinus (Plate 23, fig. 1), Pholidechinus (Plate 28, fig. 1), Lepidesthes (Plate 68, fig. 3), and Recent Phormosoma (text-fig. 43, p. 80). This last type differs markedly from those just considered in that the ambulacral basicoronal plates in the adult are compound instead of simple. No compound ambulacral plates on the peristome are known in any type. A third type of basicoronal plates is seen in clypeastroids (Echinarachnius, text-fig. 52, 72 ROBERT TRACY JACKSON ON ECHINI. p. 80) and spatangoids (Micraster and CoUyrites, Plate 3, fig. 15). In both of these the primordial ambulacral plates typically alternate with the primordial interambulacral plates to form the basicoronal row. In all other echinoids the primordial ambulacral plates are on the actinostome and form buccal plates both in the young and in the adult. Some very peculiar special modifications of this plan occur in aberrant cases in clypeastroids and spatan- goids, as follows. In Arachnoides placenta (Linne), Loven (1874, Plate 51) showed that in the young, ambulacral and interambulacral plates alternate as usual. In the adult, the posterior odd interambulacral plate has been pushed dorsally out of the basicoronal ring, and the other primordial interambulacral plates in areas 1, 2, 3, 4, have disappeared by intracoronal resorption, so that as a result, the ten primordial ambulacral plates have come in contact and alone make up the basicoronal ring. They are in contact as they are typically in the buccal membrane around the mouth in regular Echini (Bothriocidaris), but the contact in Arachnoides placenta is a secondary contact not homologous with, though analogous to the original primitive con- tact. In specimens of Arachnoides that I have examined, both smaller and larger than those figured by Loven, the primordial interambulacral plates are in place in the basicoronal row. It is probable, therefore, that Loven's larger specimen as described was an exceptional indi- vidual variant, and not typical of the species. In Lovenia forbesi Woods and Duncan, from the Miocene of Australia, the primordial interambulacral plates of areas 2, 3, 5, are in place in the basicoronal row, but the plates of 1 and 4 have been pushed dorsally so that the primordial ambulacrals 16, Ila, and lYb, \a have come in contact instead of being separated by an interambulacral plate as usual. In Pourtalesia jeffreysi Wyville Thomson, Loven (1883, p. 13) shows the extraordinary condition that the primordial interambulacral plates 2, 3 join posteriorly the base of primordial interambulacral 5 and shut out the primordial ambulacrals I, V and II, IV from the peristomal row. In this highly aberrant type, therefore, the ventral margin of the corona is made up of the two primordial ambulacrals Ilia, b, and the sum of primordial interambulacrals 2, 3, and 5, as far as known a unique condition. Dr. Mortensen (1907, p. 73, Plate 17, fig. 21) shows in Echinosigra (Pourtalesia) paradoxa Mortensen, a most excessive lengthening and also narrowing of the basicoronal plates in areas I, 5, and V, making them, perhaps, the most peculiar plates of any echinoid, in a type that takes delight in breaking all rules of echinoid structure. All the above are types of basicoronal plates in which there has been no encroachment by resorption of the ventral border of the corona. Wliere resorption has taken place to a greater or less extent, a different character of basicoronals prevails. In the Palaeechinidae resorption has apparently cut away only the single primordial interambulacral plate and at the same time, as gathered from Melonechinus (Plate 56, fig. 8), some ambulacral plates have been transferred to the peristome. In these types we find two IMBRICATION OF CORONAL PLATES. 73 plates at the ventral border of the interambulacrum and two in the ambulacrum (Maccoya, Plate 33, fig. 1; Lovenechinus, Plate 43, fig. 1; Oligoporus, Plate 50, fig. 8), or two in the interambulacrum and four in the ambulacrum (Melonechinus, Plate 56, fig. 2). Resorption may have cut away more than one row, probably three or more rows, when we have the condi- tion in Archaeocidaris (Plate 9, figs. 7, 8; Plate 10, fig. 10), where there are four plates in the interambulacrum and two in the ambulacrum in the basicoronal row. Resorption may cut away an indeterminate number of plates, when we find the condition seen in most modern regular Echini, as Cidaris and Strongylocentrotus, with two interambula- cral and two ambulacral plates in each area in the basicoronal row. This includes, I believe, the essential structure of the basicoronal row of all Echini. The proximal modifications of these plates by the development of the perignathic girdle are considered under that head (p. 190). The enlargement of the peristomal aperture in Echini is attained l^y two methods: by the growth of the individual plates, which will enlarge the opening to any extent ; or by cutting away the plates by resorption, which is a more obvious method of enlargement. The growth of the plates is, however, the more important factor. Even when resorption has not cut away any interambulacral plates, as in Echinarachnius, Spatangus, Arbacia (text-fig. 227, p. 193), and especially Phormosoma (text-fig. 43, p. 80), the size of the peristomal opening is many times larger than it was in the young individual. Imbrication of Coronal Plates. In Echini, in many types, there is no imbrication in the coronal plates, the suture lines of the plates being so nearly perpendicular to the surface that the sides of a plate are as nearly parallel as possible in a curved test. The sides of a plate may be far from -parallel where the plates are very thick, as in Lovenechinus nobilis (Plate 38, fig. 9), but still they are perpendicular to the surface and are therefore not imbricate. Certain plates in Echini may present a strong angle on the suture line in a given direction, which Bather suggestively calls beveling. Beveling is applied as a term where the inclination is in a limited number of plates only. An excellent example is in the adradial suture of the Palaeechinidae. Here in all species of the family, as shown in Lovenechinus septies (Plate 45, fig. 5), the ambulacral plates are inclined over the adjacent interambulacral plates, so that a strong bevel results in the plates concerned. A similar bevel is seen in the petaloid area in some clypeastroids (Clypeaster), where the ambulacra are strongly inclined over the interam- bulacra on the adradial suture. In Clypeaster rosaceiis a curious combination of direction of beveling occurs. As stated, in the petaloid areas, the ambulacra bevel over the interambulacra as they do in the Palae- 74 ROBERT TRACY JACKSON ON ECHINI. echinidae. On the other hand, ventrally in Clypeaster, the interambulacra bevel over the ambulacra (text-fig. 234, p. 197) as in Pholidechinus or other types that have imbricate plates. A succession of bevels, in a meridional or transverse series, as Bather (1909a, p. 64) says, produces imbrication properly so called. Imbrication is a character of many quite independent series in Echini, and while of genetic value in a limited series, cannot be used as a basis of close systematic connection in Echini as a whole, as I have shown (1896, p. 237). As imbrication is confusing, especially in fossils where specimens are often viewed from the internal side; diagrammatic figures are given (text-figs. 32-39), illustrating the chief char- acters. Imbrication in Echini is curiously constant in its direction. Interambulacral plates in the corona always imbricate aborally in the vertical plane, so that the dorsal border of one plate is inclined over the ventral border of the next adjacent dorsal plate of its series (text-fig. 32). Interambulacral plates also always imbricate outwardly or adradially from the center, so that each plate from the center of the area laterally imbricates over its neighbor toward the ambulacral area and on the adradial suture imbricates over the ambulacral plates, as seen in Lepidesthes wortheni (Plate 67, fig. 8). In the ambulacrum, on the contrary, the imbrication is always adoral, so that the ventral border of one plate inchnes over the dorsal border of the next adjacent ventral plate, in the opposite direction from that of the interambulacral plates (text-fig. 38). Laterally there is little or no imbrication in ambulacral plates (Lepidesthes) except on the adradial suture, where the ambulacral plates dip under the adjacent interambulacrals instead of over them, as they do in the Palaeechinidae and in some clypeastroids, as above noted. The imbrication of ambulacral and interambulacral plates, as described, occurs weakly in Archaeocidaris, Lepidocidaris, Lepidechinus, strongly in Lepidocentrus (Plate 20, fig. 8), Hyattechinus (Plate 22), Pholidechinus (Plate 28, fig. 2), Perischodomus (Plate 64, figs. 2, 3), Lepidesthes (Plates 66, 70), Pholidocidaris (Plate 73, fig. 4), and Meekechinus (Plate 76, fig. 1). Imbrication also occurs in the modern thin-plated Echinothuriidae. This character has been considered a basis of connecting the Echinothuriidae with Palaeozoic types having imbri- cate plates, but so many characters are opposed to it, that this must be considered a case of parallelism. In modern forms, imbrication is not limited to the Echinothuriidae, but occurs also in the Centrechinidae. This character is well developed in Aslropyga pulvinata (text-fig. 100, p. 109), where interambulacral plates iml^ricate dorsally and laterally, and the ambulacral ventrally. The plates are so thin that it is difficult to see the sutures of imbrication in cross section, but the imbrication is readily seen on the internal view of the test. The interambula- cral plates imbricate over the ambulacrals, as they do also in echinothuriids, in Centrechinus, and in the Palaeozoic types with imbricate plates (as noted ii»the Palaeechinidae, the ambula- cral plates are beveled over the ijiterambulacrals, but imbrication proper does not occur in this family). Mr. Agassiz (1881, p. 71) notes the occurrence of imbrication in Astropyga, but IMBRICATION OF CORONAL PLATES. 75 Text-figs. 32-39. — Method of imbrication of plates in Echini; the figures 32-38 are sj'nthetic, based on studies of Lepideslhes colktii (Plates 69-71). 32. External view, interambulacral plates are rounded in outline. Column 3 imbricates dorsally and laterally in two directions. Columns 1, 2, 4 imbricate dorsally and laterally in one direction and over ambulacrals. Ambulacral plates imbricate ventrally, bevel under adjacent interambulacrals, but do not otherwise have lateral beveling. 33. Cross section of the same. 34. Internal view of the same. Interambulacral plates are hexagonal in outline, angles of imbrication reversed from external view, ambulacral plates with pores in the middle of plates, instead of near the intcrambulacrum as in external view. 35. Cross section of same. 36. Optical section of plates seen from exterior. 37. Optical section of plates seen from interior. 38. Showing in vertical section direction of imbrication of plates in the interambulacrum and ambulacrum. 39. Eucidaris tribuloides {La,ma,Tck). Bahamas. R. T. J. Coll., 693. X 4. Base of corona and peristome. On the peristome both ambulacral and non-ambulacral plates imbricate adorally, a condition not known in the corona of any echinoid. 76 ROBERT TRACY JACKSON ON ECHINI. says the plates of both areas imbricate in the same direction dorsally. I have studied imbri- cation in only one specimen, an Astropyga pulvinata; but in that one the ambulacra imbricate ventrally, interambulacra dorsally as described. I believe the fact has been overlooked, but a weak imbrication occurs in Centrechinus setosus. It is not obvious in external view, but it is readily seen on viewing the interior of a test. The interambulacral plates imbricate dorsally as usual, and the ambulacral slightly ventrally; the interambulacra are inclined over the ambulacra on the adradial suture as usual in imbricate Echini, living or fossil. If a specimen of Centrechinus from alcohol is sectioned horizontally by cutting through the ambitus, it is found that there is much flexibility of the test. The same is seen in a dried specimen if it is first soaked in water. In Chaetodiadema the test is thin and highly flexible. It is very interesting that flexibility and imbrication are so charac- teristic of the Recent Centrechinidae, for many characters connect this family with the Echinothuriidae, as clearly shown by Messrs. A. Agassiz and Clark (1909). The rule of ventral imbrication for ambulacral and dorsal for interambulacral plates in the corona, is apparently without exception in the corona for all Echini where imbrication occurs. Certain exceptions according to the literature are said to exist. Worthen and Miller (1883) supposed that in Lepidesthcs ( = Hybochinus) spectabilis the interambulacral plates imbricate ventrally and the ambulacral dorsally. Their specimen was doubtless incorrectly oriented on account of the position of the teeth as discussed under the systematic description of that species, so that if I am correct it is no exception to the rule. In my earlier paper (Jackson, 1896, pp. 223, 224) a serioys error was made in regard to Lepidocentrus mulleri where the interambulacra were described as imbricating ventrally; this was wrong, since the specimen was inverted, as discussed under that species in this paper. Again, an error was made in describing Pholidocidaris meeki (1896, p. 211, Plate 9, fig. 54). The interambulacra and ambulacra were described as both imbricating adorally. The speci- men (Plate 73, figs. 6, 7) is imperfectly preserved, and I can only plead guilty of an error of observation. I was also partly misled by the fact that Meek and Worthen's (1873, Plate 15, fig. 9a) figure of Pholidocidaris irregularis, the only good figure published, was inverted, a fact that was since ascertained. There is no proof at present that imbrication of coronal plates is ever other than dorsal in the interambulacra and ventral in the ambulacra in types of Echini with imbricate plates. On the peristome, imbrication is adoral when it exists in both ambulacral antl interambula- cral plates, as in Eucidaris (text-fig. 39, also text-figs. 43, 46, 47, p. 80). When imbricate specimens of fossil Echini are viewed from within (text-fig. 34), the struc- ture appears very different from that as seen from without (text-fig. 32). This is shown well in Perischodomus biserialis (Plate 62, figs. 6, 7; Plate 64, figs. 2, 3). From the interior, the ambulacra appear to imbricate dorsally and laterally overlap the adjacent interambulacra; SPINES. 77 also the ambulacra are wide internally instead of narrow, on account of their lateral beveling. The interambulacra appear to imbricate ventrally, as seen from the interior, and dip under the adjacent ambulacra. AH this is reversed when the specimen is viewed from the exterior. This complication must be constantly borne in mind in a study of internal and external molds of fossils or confusion will ensue. Spines. Having discussed the corona, a brief consideration may be given to spines as an obvious and salient feature of Echini. The name Echinodermata implies the character of spines. It is a misnomer, if the group as a whole is considered, for in the Pelmatozoa, spines are rarelj^ developed, and in the holothurians are absent. Spines are, however, a feature of the Echini, and there is no known species which is without them. In the very young plates at the dorsal area of the test, spines are absent, not yet having developed. The nearest approach to this condition as a species character is seen in Bothriocidaris pahleni (Plate 1, fig. 3), which in the interambulacrum apparently has no spines in the adult. It is true that minute granules exist, which possibly bore spines, but such are not known. This species, however, has spines on the ambulacra and all other Echini have them on the interambulacra as well. Spines are classed as primary, secondary, and miliary, and are attached to tubercles of corresponding size. In the Palaeozoic, primary spines are especially well developed in Archaeo- cidaris, where in each interambulacral plate there is a single large central perforate tubercle with an associated spine of variable but always relatively large size. In the youngest plates at the dorsal area, as shown in Archaeocidaris rossica (Plate 11, fig. 2), neither tubercles nor primary spines are developed, and when the tubercles develop, they are at first imperforate as seen in the younger dorsal plates in the figure cited. In Eucidaris (Plate 3, figs. 1, 2) a similar condition exists: the young tubercle is imperforate and the young spine is a thin hollowed plate, like an inverted saucer, its spine-like character developing in later growth. A young spine of a cidarid is short, broad, and distally rounded, and reminds one of the character of the spines of Colobocentrotus, except for the absence of marginal angles commonly developed in the spines of that type by mutual pressure. Central primary spines are well developed in Lepidocidaris (Plate 16, figs. 1-3). Spines of larger size than associated spines and which may be fairly called primaries, are found irregu- larly distributed in Lepidocentrus (Plate 19, figs. 3-5), in HyaUechinus rarispinus (Plate 23, fig. 6), and on the adambulacral columns in Pholidocidaris (Plate 73, fig. 3), and, judging from the tubercles, in part in Perischocidaris (Plate 67, fig. 2). In the several types considered, there are also small secondary spines clustered around the primaries and alone existent on the ambulacral plates (Archaeocidaris). Or secondary spines and tubercles may occur with pri- 78 ROBERT TRACY JACKSOX ON ECHINI. maries on adambulacral columns and exist alone on other interambulacral colunuis and on the ambulacrum (Pholidocidaris, Plate 73, fig. 3; Perischocidaris, Baih', 1874, Plate 4). Secondary spines may exist without any primaries. Such is the condition in all the Palae- echinidae (Plate 52, figs. 10, 11), in Lepidesthes (Plate 70, fig. 4), and, judging from the small and uniform size of the tubercles, in Lepidechinus (Plate 63, fig. 3) such spines and tubercles may be fairly compared in size to those characteristic of Clypeaster and the usual type seen in clypeastroids. Such small spines as occur in Bothriocidaris (Plate 1, fig. 3) may probably be considered as primaries on account of the perforate tubercles and the small .size of the whole animal. As far as known, spines are very uniform in character within the species in the Palaeozoic, cases of marked deviation, such as occur in some Cidaridae, being almost unknown in these older types. From this uniformity in the Palaeozoic, spines present very good species characters, which is fortunate, as they so often occur in a dissociated condition among fossils. As discussed (p. 51), spines are typically internal, and the evidence indicates that they were internal, that is, covered by living epithelial tissue, in all Palaeozoic types excepting Miocidaris and possibly Archaeocidaris. Stout, thick, and club-shaped spines are largely characteristic of the Cidaridae and a few other types. A specimen of Strongylocentrotus drobachiensis in the Peabody Museum at Salem is interesting as showing thick club-shaped spines (Plate 6, figs. 11, 12), which are quite cidarid in appearance. These spines are numerous but rather local in distribution, not covering the whole specimen. The specimen is of normal size and appearance otherwise, and it appears to be merely a rare variation. This was the only such case seen in 33,000 specimens of this species examined, nor was such a structure seen in any other species of the family. Tubercles as well as the spines with which they are associated may be divided into primary, secondary, and miliary. Primary tubercles may be perforate or imperforate. They are perforate in all Palaeozoic species where they occur; they are also perforate in the Cidaridae, Centrechinidae, and some of the Saleniidae, also in some spatangoids. Primary tubercles are imperforate in the remaining Centrechinoida, the Holectypina, and many of the Spatangina. Secondary tubercles are imperforate in the Palaeozoic, whether they occur with primaries or alone, as in the Palaeechinidae. They are also imperforate in the Cidaroida and in Acrosalenia, but they are perforate in the Centrechinidae and certain clypeastroids and spatangoids (Clypeaster, Metalia, Meoma). Perforation of tubercles is evidentlj^ a character of primitive types and of some specialized types of clypeastroids and spatangoids. Primary tubercles in a species occur with a definite distribution which is characteristic, as seen especially well in the Centrechinoida, a given number typically existing on a plate. It seems that the distance between tubercles which is maintained in a species is the real feature of their distribution. If, by chance, a plate is narrower than usual, as in Tripneustes (Plate 6, fig. 3), the tubercles are their usual distance apart, but there are fewer to a given plate; THE PERISTOME. 79 or, if, by chance, a plate is wider tlian usual, as in the tetramerous Arbacia lixula figured by Verrill (1909), then again the tubercles are their usual distance apart, but there are more tubercles to a plate than is typical of the species (pp. 40, 47). The Peristome. The peristome is closely associated with the corona, and is next considered. The peris- tome, which is the tissue lying between the mouth opening and the basicoronal plates, presents various characters in different groups of Echini, and these characters fall into six quite distinct types as described below. The peristome in almost all Echini is more or less extensively covered with plates. There are in regular Echini ambulacral plates bearing tube-feet, either in one or in many rows. With these there may be in addition what I would call non-ambulacral plates, which do not bear tube-feet. Such non-ambulacral plates, where they occur in the Perischoechinoida and Cida- roida, occupy an interradial position ; or, where they occur in the Centrechinoida, they occupy also a radial position, being a more or less generally distributed series of plates (Toxopneustes, text-fig. 57). In spatangoids there are no ambulacral plates on the peristome, but non-ambula- crals only. These are generally distributed over the area. I have adopted the term non- ambulacral for such plates because, though they may be interradial only (Cidaridae), they may also have a general distribution, and they have no relation to the interambulacra of the corona. The most ancient type of peristome known is that represented by Bothriocidaris (text- fig. 40) in which there are two continuous rows of ambulacral plates, a structurally primitive character. The first row around the mouth can fairly be considered the primordial ambulacral plates, and, as shown in B. archaica (Plate 1, fig. 1), this row follows Loven's law of la, Ila, III6, IVa, V6 large and 16, lib, Ilia, IV6, Va small. Above the second row the corona proper begins with a row of five interambulacral and ten ambulacral plates. The line of separation between the peristome and the corona is not so strongly marked as in most types, but it is comparable to that of young Phormosoma (text-fig. 41), as pointed out by Mr. Agassiz (1904, p. 79). It is quite probable that the second row of peristomal plates in Bothriocidaris was derived by flowing down from the corona, but on the other hand there may have been two rows of peristomal plates formed before the development of the corona. In Phormosoma at a very young stage (Plate 3, fig. 9), as shown by Mortensen (1904, p. 54), there is only one row of iK'ri- stomal plates, whereas a little later (text-fig. 41; Plate 3, fig. 10), as shown by A. Agassiz, there are two rows of peristomal plates almost exactly as in Bothriocidaris, as Mr. Agassiz (1904, p. 79) pointed out. The peristome of Bothriocidaris makes also very close approach to the character of the young in cidarids (Plate 2, figs. 1, G) and young Strongylocentrotus (text-fig. 49), also other young Centrechinoida. It differs from these early stages principally in having 80 ROBERT TRACY JACKSON OX ECHINI. ^ nL THE PERISTOME. 81 Text-figs. 40-54. — Characters of peristome and base of the corona in repre.sentativc Ecliini. 40. Jiolliiiocklaris tirchiiica sp. nov. Ord()\ician, Russia. From Plate 1, fig. 1. On the peristome two rows of ambu- lacral plates. 41. t'lioniKixdiiKi iiliiccnlaWyvWU' TliomMm. Young. Diani. 9 nun. Adapted from A. Aga.ssiz, 1004, Plate 43, fig. 3. On the ])eri.slomc' two rows of ambulaeral plates. 42. The same. Diam. 7 mm. .\dapted from A. Agassiz, 1904, Plato 43, fig. 1. Peristomal jilates with pores in a central iierijiodium. 43. The same, adult . Off Cape May to Cape Sable, 956 fat h. Diani. 50 mm. R. T. J. Coll., 707. X 3. On the peristome many row's of ambidaeral plates. 44. The same. Peristomal gills enlarged. 45. (•oniocidaris canalicitlala A. Agassiz. Young. Diam. 1.45 mm. From Plate 2, fig. 1. On the peristome one row of ambulaeral plates. 40. Eiicidari.-i Irihidoiihs (hamiirck). Bahamas. Diam. 45 mm. R. T. J. Coll., 708. X 3. On tlie jteristome many rows of ambuhirnd ami in addition intcrradial non-a,inl)ulacral plates (compare with young, Plate 2, hg. 0). 47. Airhiicucidaris wnrlkini Hall. Lower Carboniferous. From Plate 9, fig. 0. Partially' restore > m C S 1 o QJ t—i )-H HH t~^ > »— f l-H i" > "3 u O 1— t 1— I l-H > g > 1— T 1 Q ■4-3 c l-H t-H .9 > l-H > 1— T 2 O l-H l-H HH OJ .9 <— ( ►— ( > hH"" £ o U QJ HH HH HH HH HH >' > hh" 2 o O C 2 3 O < a c3 •i £ 3 S "o a CD a tJ) c rt t.. (-1 < a> a> CD 0} g 1— 1 hH ,_( HH. HH h-i t—i 1— 1 kt t-H t-H HH ^ HH & - H- T »— T •—T ^'' -9 t— < 1—1 H- 1 HH Ch ci 03 m m > > (—4 > 1— 1 o gj «; t-^ 3 .a 'G > -t-3 1 > 1— 1 > QJ o OJ 9i ii QJ .s •— 1 •— I M o P4 Si > > > 0) o M t— I > 1— r 1— r h-l a S3 "3 £ s £ QQ "B 0) *n o -2 _C3 _rt u a 1 o o C3 "5 3 "3 3 =) Fh o o o o O t-t !? > o c )— ( t— 1 > Li > 1— 1 fc-i a; C > > 1 "3 o +^ ^ ^ 1 "3 o o < 1— 1 "3 o O > O 3 o O 2 "3 o 1— 1 o O o a 1 50 Gullmar Fjord on the Swedish coast of Skagerrak SO ss 12 15 29 6 1,700 Pulpit Harbor, North Haven, Me., 30-.58 mm. diam. 10 O.Off 88 0.5 ; 173 1 1,503 9 14 194 St Pierre Newfoundland 6 9;g 0.5 2 12 178 1 3 Dumpling Islands, North Haven, Maine. (This series, given above, belongs here.) 6,0.50 developed g.5 0.03 95 /.; 2 specimens, 30-75 mm. diam. 7,600 Calderwood Island, Fox Island Thoroughfare, Maine 3.2 0.07 H 1.6 0.03 1 242 5 7,157 118 2 76 2,700 York, Maine (text-fig. 176, p. 153) 0.9 0.04 S7 ^ 0.04 1 25 1 2,610 38 1 25 1,200 Friday Harbor, Puget Sound, Washington, 40-85 0.9 96 g.e 0.1 / mm. diam. 11 1,149 31 1 8 588 Massachusetts Bay, mostly from Nahant, Mass. 14 95 2.2 0.^ 2 8 557 13 1 9 4,000 Frenchman's Bay, Maine 0.03* 1.9 94 ;J.3 0.05 / 1* 75 3,745 130 2 47 300 South Harpswell, Maine 2.7 0.3 92 3.7 ; 8 1 •271 11 3 Truro, Massachusetts, series 20-30 mm. diam. (1,100 specimens) 0.3 96 .3.5 1 2,800 3 1,0.52 32 13 Same locality, series 30-60 mm. diam. (1,700 speci- O.G H 4.3 O.Off / y mens) 11 1,595 73 1 20 200 Labrador 2.5 0.5 S7.5 7.5 2 T* 1 175 15 4 35 Iceland and Faroe Islands 11 eo ao .9 4 21 y 3 33,000 20 1,758 11 30,231 569 9 402 ' Italic numerals represent percentages, Arabic numerals the number of specimens observed. 144 ROBERT TRACY JACKSON ON ECHINI. pores and ocular I only is insert; an older, but still very young specimen, 22 mm. in diameter, has the bivium insert, the species character. Of adults, 179 were examined, including a superb series of 61 specimens from Friday Harbor, Puget Sound, Washington, kindly sent me by Mr. A. P. Romine. Of this species, in 87 %, oculars I, V are insert (text-fig. 152). In 1 % ocular I alone is insert as an arrested variant. Both of the two specimens showing this character are adults, one being very large, 145 mm. in diameter (text-fig. 151). Of progressive variants Text-figs. 150-153. — Ocular plate arrangement in Slrongylocentrotus franciscanus A. Agassiz. 150. Monterey Bay, California. Young. Diam. 5.5 mm. R.T. J. Coll., 552. X 6.5. Ocular I alone insert as a developing character, no genital pores. 151. Friday Harbor, Puget Sound, Washington. Diam. 145 mm. R. T. J. Coll., 906. X 1.6. Ocular I alone insert, an arrested variant. 152. Cahfornia. Diam. 154 mm. R. T. J. Coll., 724. X 1.6. Oculars I, V insert, the tjpical character. An exceptionally large specimen (text-figs. 166, p. 149; 199, p. 171). 153. Vancouver, British Columbia. Diam. 126 mm. R. T. J. Coll., 725. X 1.6. Oculars I, V, IV in.sert, a progres- sive variant. 11 % have oculars I, V, IV insert. The specimen from which text-fig. 152 was drawn is ex- ceptionally large, 154 mm. in diameter, the largest seen, yet it has only the bivium insert; the progressive variants were all considerably smaller. One specimen only is aberrant with oculars V, II insert, ocular I being excluded from the periproct by the fusion of genitals 5, 1 as in text-fig. 146. Of the three adults here figured it is noteworthy that the largest specimen (text-fig. 152) has the smallest apical disc and the next smaller specimen, which is an arrested variant (text-fig. 151), has the largest apical disc (p. 87). The most progressive species of the genus is Slrongylocentrotus purpuralus from California. Through the kindness of Dr. W. K. Fisher, I received a fine series of specimens; with others, 120 have been studied. In this species 77 % have oculars I, V insert, 1 % is arrested with I only insert, and 22 % are progressive variants with I, V, IV insert. The species eury- throgrammus, franciscanus, and purpuratus, although the\' have oculars I, V insert as a typical OCULAR AND GENITAL PLATES. 145 character, have a larger number of progressive variants as regards ocular plates than any other species of the genus (table, p. 162). Strongylocentrotus gibbosus is a most unusual sea-urchin in that it has deeply bedded in the periproct a commensal crab, Fabia chilensis Dana. This crab is a very constant associate; but one specimen in 24 studied has no trace of a crab (text-fig. 155). The anomalous associa- tion of the crab very deeply modifies the relative position of the ocular plates. The crab in the cases seen occupies a position close to ocular IV and genital 3, as shown in the figures, and no variation was noted in this position. This species and Gymnechinus are the only Echini studied in which an aberrant arrangement of ocular plates is the dominant one. In 24 specimens one has I, V., IV insert (text-fig. 154), and four have all the oculars insert 756 Text-figs. 154—157. — Oeiilar plate arrangement in Sirongylocenlrotiis giJihosus (Agassiz). Payta, Peni. 'I'lie tyjiical commensal crab, Fabia chilensis, in all but text-fig. 155. All figures X4. 154. Diam. 31 mm. R. T. J. Coll., 704. Oculars I, V, IV insert. 155. Diam. 28 mm. R. T. J. Coll., 702. Oculars V, IV insert. Crab wanting. 156. Diam. 42 mm. R. T. J. Coll., 703. Oculars V, IV, III in.sert, the typical character in tliis aberrant species; the periproct meets the interambulacra at the points marked X. The commensal crab is very clear and in its (ypical posi- tion (pp. 94, 128, 177). 157. Diam. 40 mm. R. T. J. Coll., 705. Ocular plates all insert. (text-fig. 157). These are the only cases of all oculars insert seen in the genus. It is also interesting that in text-fig. 157 there is an interspace between ocular III and genital 3, where the periproct reaches the interambulacrum, and the same feature is shown in three areas in text- fig. 156. This is comparable as a parallelism to a similar condition of separation of ocular and genital plates in Phormosoma (p. 110). All the other specimens, 79 %, are aberrant in ocular arrangement. In five, oculars V, IV are insert; one of these has no associated crab (text-fig. 146 ROBERT TRACY JACKSON ON ECHINI. 155), but yet retains the specific irregularity. Eight have V, IV, III insert (text-fig. 156), a very unusual arrangement, seen in only three other cases in the order Centrechinoida, yet it is the dominant character, 33 %, in S. gibbosus. This arrangement of oculars with V, IV, III insert was found as an aberrant variation in six cases in the Cidaroida, as described in Cidaris affinis and Eucidaris tribuloides (pp. 96, 97, 99). Six specimens, 25%, have oculars I, V, IV, III insert, again a rare arrangement in the order, but frequent in the Cidaroida. In young specimens of gibbosus a few mm. in diameter all oculars are exsert. In the Strongy- locentrotidae, of the species studied, two are characterized by having oculars all exsert; all others have typically the bivium insert iexcept the aberrant S. gibbosus (pp. 162, 164). The Echinometridae as here restricted includes Echinometra and allies, which are elliptical in outline through a sidewise axis, a character peculiar to this group of Echini only. In this family, as in the Arbaciidae, when ocular plates enter the periproct, it is in the sequence V, I, not I, V, as ill the Echinidae and Strongylocentrotidae. On this basis, when only one ocular is insert, it should be V, and this is true as a dominant rule; occasionally, however, ocular I alone is insert. Such cases may be considered variants toward the character of the associated families in which, if only one plate is insert, that plate is typically ocular I. The rule is not quite so constant as in the Arbaciidae, in which, if one plate is insert, it is ocular V with very rare exceptions. The most primitive species of Echinometra on the basis of ocular plate arrangement is E. oblonga. Of this species (44 specimens) in 98 % all the oculars are exsert, and in 2 % ocular V is insert as a progressive variant. In Echinometra viridis (25 specimens), 96% have all oculars exsert, and 4% ocular V insert. Echinometra mathaei {15Q specimens) has typically, 79 %, all oculars exsert; of progressive variants 19 % have V, and 1 % have V, 1 insert; 1 % is aberrant with oculars V, II insert. Of Echinometra lucunter a large series was studied, and it shows different facies in two widely separated localities. In the West Indian area (578 specimens) typically, 57 %, ocular V alone is insert (text-fig. 159). As arrested variants 17 % have all oculars exsert (text-fig. 158), the typical character of E. oblonga. As progressive variants 25 % have oculars V, I insert (text-fig. 160), like the typical character of E. van brunti, and 0.3 % have V, I, IV insert (text-fig. 161). In Echinometra lucunter from Bermuda considerable difference exists. In 176 specimens from that locality ocular V is typically insert in 46 %. The arrested variants, however, are less frequent, only 9 % having all oculars exsert. Also the progressive variants are much more frequent than in the West Indian form, there being 43 % with V, I, and 0.6 % with V, I, IV insert. The Bermudan form, while having typically ocular V insert as in the West Indian, is therefore distinctly more progressive as shown by its variants, and makes a closer approach to the character seen in the next higher species, E. van brunti. The Bermudan form of E. lucunter is notable for its large size and robust character. One specimen in my collection from that locality measures 90 mm. through the long axis. Wliile in Echinometra OCULAR AND GENITAL PLATES. 147 lucunter 413 specimens have ocular V alone insert, only four have ocular I alone insert, showing the strong dominance of this character. In the whole number (754) examined, there are but two specimens, 0.3 %, with an aberrant arrangement of oculars. In one, oculars V, IV are insert, and in the other V, II alone reach the periproct (tables, pp. 163, 164). Text-figs. 158-161. — Ocular plate arrangement in Eddnometra lucunlcr (Linn6). Hayti, West Indies. All figures X4. (Compare text-figs. 111-114, p. 115.) 1.58. Length 41 mm., through 3, I. R. T. J. Coll., 768. Oculars all exsert, an arrested variant. 159. Length 55 mm., tlirough 3, I. R. T. J. Coll., 769. Ocular V insert, the typical character. 160. Length 52 mm., through 3, I. R. T. J. Coll., 770. Oculars V, I insert, a progressive variant. 161. Length 41 mm., through 3, 1. R. T. J. Coll., 771. Oculars V, I, IV insert, an extreme progressive variant. Echinometra van brunti is an interesting form as it is apparently the highest species of the genus, judging from the ocular plates. In 76 specimens the typical character is to have oculars V, I insert, which occurs in 72 %. As arrested variants only 11 % have oculars all exsert, which is the character of E. mathaei; in 12 % V only is insert, which is the character of E. lucunter; and in 4 % ocular I only is insert. As a progressive variant, 1 % have V, I, IV insert. It would be most interesting to get a large series of this species for comparative study, but it is felt that the number seen is enough to give a fair basis for comparison. In Heterocentrotus mammillatus (35 specimens) 80 % have oculars all exsert, 11 % ^^re pro- gressive variants with ocular V insert; in one specimen ocular I, and in one oculars V, I are insert. One specimen is aberrant with oculars V, II insert. In the nearly allied Heterocentrotus trigonarius (47 specimens) typically, 60 %, all oculars are exsert; in 28 % ocular V is insert, in 4 % ocular I and in 9 % oculars V, I are insert. In Colobocentrotus atratus (82 specimens) 99 % have all oculars exsert, and in 1 % ocular V is insert, as a progressive variant. The Echinometridae as a family is characterized by very close adherence to the rules of ocular arrangement. In the whole 1,222 specimens examined, only four aberrants were found; of these, three have oculars V, II and one has V, IV insert (tables, pp. 163, 164). Having considered the character of ocular plates in their relation to the genitals and to 148 ROBERT TRACY JACKSON ON ECHINI. the periproct as typical developing and adult features, and as variants in many species of fossil and living Echini, the characters may be summarized as follows. In the Ordovician Bothrio- cidaris archaica the oculars form a continuous ring with genitals on their dorsal border (text- fig. 162). In the Devonian to Carboniferous usualty, as in Melonechinus (text-fig. 163), oculars and genitals all reach the periproct and the corona. In the Mesozoic usually, as in Cidaris coronata (text-fig. 164), the genitals alone reach the periproct and oculars are all exsert. In the Recent, oculars are all exsert in the young individual. In the adult typically all oculars may still be exsert, or one, or more to all plates may travel in so as to reach the periproct, and the incoming, barring relatively rare exceptions, is in a perfectly definite order. Different species or genera attain as a character definite points along this line of dilTerential development, and selected cases of coming in are shown in text-figs. 165 to 169. All plates may be typicallj' exsert, as in Echinus escidentus (text-fig. 116, p. 117). One plate may be typically insert, when it is either ocular I, as in Echinus niagellanicus (text-fig. 165), or ocular V, as in Echino- Text-figs. 162-175. — Ocular plate arrangement in typical Echini. Each figure represents the typical character of the several species represented. 162. Bothriocidaris archaica sp. nov. Ordovician, Russia. From Plate 1, fig. 2. X 7. Oculars very large, meeting in a ring, genitals small, dorsal to the oculars (p. 87). 163. Melonechinus mulliporus (Norwood and Owen). Lower Carboniferous, St. Louis, Missouri. From Plate 50, fig. 6. X 3. Oculars and genitals all meet the periproct and corona. Compare text-fig. 169. Genitals have many pores. 164. Cidaris coronata Goldfuss. White Jura, Sonntheim. Stuttgart Museum 9,782. Enlarged. All oculars exsert. Plates of periproct in place, very rare for fossils of the genus (pp. 96, 174). 165. Echinus magellanicus ThiWppi. Patagonia. Diam. 26 mm. R. T. J. Coll., 773. X 2.7. Ocular I insert (p. 119). 166. Slrongylocentrotuit franciscanu-s A. Agassiz. California. Diam. 112 mm. R. T. J. Coll., 699. X 1.7. Oculars I, V insert (p. 144). 167. Arbacia nigra (Molina). Chili. Diam. 92 mm. R. T. J. Coll., 796. X 2.6. Oculars I, V, IV insert (p. 116). 168. Acrosalenia pseudodecorala Cotteau. Bathonien, France. (After Cotteau, 1S7.5-'S0, Plate 246, fig. 6.) Oculars I, V, IV, II insert. Suranal plate is dorsal to genital 3 (p. 112). 169. Phyllacanlhusbacidosa (Lamarck). Mauritius. Diam. 41 mm. R. T. J. Coll., 695. X 2.7. All oculars insert. (Compare text-fig. 163, p. 102; Plate 3, figs. .3, 4.) 170. Phortnosoma placenta Wyville Thomson. Off Cape Sable to Cape May, 956 fath. Diam. 56 mm. R. T. .1. Coll., 707. X 3.5. All oculars insert and interspaces exist between oculars and genitals so that the jieriproct reaches the interambulacra. Genitals are split ventrally. Ambidacral plates are simple primaries in the placogenous zone. Inter- ambulacral plates originate again.st oculars as iisual (pp. 63, 110, 177; Plate 3, fig. 8). 171. IIolectypHS hemisphericus Desor. Inferior Oolite, Leckhampton Hill, England. Diam. 28 mm. R. T. J. Coll., 738. X 9. Oculars all excluded from the center by genitals (p. 170). 172. Cassidulus subquadratus Conrad. Upper Cretaceous, Holly Springs, Mississippi, .\fter W. B. Clark, 1893, Plate 31, fig. Ih. Enlarged. Oculars I, V reach the center, other oculars are excluded from the center by the genitals. 173. Toxobrissus pacificus A. Agassiz. Adapted from A. Agassiz, 1904, text-fig. 279, p. 193. Enlarged. Oculars I, V are virtually insert, although actually ocular I is shut out by the posterior extension of genital 2. 174. Micrnslercorangiiineum {Lamarck) . Cretaceous, England. Length 55 mm. R. T. J. Coll., 521. X 6.7. Oc\i- lars I, V, IV reach the center, others excluded from the center by the genitals. Compare text-fig. 167 (p. 92). 175. Ananchyles ovalus (Leske). Cretaceous, Sussex, England. Length 67 mm. Student Laboratory Coll., 172, Harvard University. X 2.4. Oculars I, V, IV, II reach the center, III only being excluded from the center by the genitals. Compare text-figs. 126 and 168 (pp. 92, 167). OCULAR AND GENITAL PLATES. 149 150 ROBERT TRACY JACKSON ON ECHINI. ynetra lucunter (text-fig. 159, p. 147). Two plates may be insert, when it is typically the bivium, as in Strongylocentrotus franciscanus (text-fig. 166). Three plates may be insert, when it is the bivium and left posterior plate of the trivium, as in Arbacia nigra (text-fig. 167). When four plates are insert, it is typically the bivium and posterior pair of the trivium, as in Acrosalenia pseudodecorata (text-fig. 168). I have shown this character in many cases as a variant but have not seen a species in which it is the character, so have copied the figure from Cotteau (1875-'80), as he says that four plates are typically insert in that species. All ocular plates may be insert, as in Phyllacanthus baculosa (text-fig. 169), when we have a character which is a close approach to that dominant in the Upper Palaeozoic (text-fig. 163). The next differential character to appear is for interspaces to develop between the ocular and genital plates, so that the periproct comes in contact with the interambulacra, as in Phormo- soma placenta (text-fig. 170). Turning to the irregular Echini, we find in part a parallel series to that seen in the regular Echini. As the periproct is eccentric, of course oculars cannot reach that structure, but if oculars reach the center line, they may be considered insert, or if excluded from the center line by junction of the genital plates, they may be regarded as exsert. Oculars may be all oxsert, as in Holectypus (text-fig. 171; compare text-fig. 164). The bivium alone may be insert, as in Cassidulus (text-fig. 172; compare text-fig. 166). In some species, as Toxobrissus (text-fig. 173), the plates of the bivium may be separated by the posterior extension of the madreporite, but as this plate radially speaking is out of place, the oculars I, V may still be considered as morphologically insert. Or it could be looked at from another point of view and ocular V alone regarded as insert (as is common in the Arbaciidae), the other four oculars being shut out from the center by the contact of the genitals. Three ocular plates may be insert, when it is the bivium and left posterior plate of the trivium, as in Micraster (text-fig. 174; compare text- fig. 167). Or, finally, four ocular plates may be insert, when it is the bivium and posterior pair of the trivium, as in Ananchytes (text-fig. 175; compare text-fig. 168). The typical character of arrangement of ocular plates and the range of variation are ex- pressed diagrammatically by block lines in text-fig. 176. Each block represents the frequency of the character in the species, as given in the tables (pp. 100, 101, 143, 154-163). An excep- tion to this statement is in the cases of Strongylocentrotus lividus (p. 126) and Phyllacanthus annulifera (p. 102) in which additional specimens, tabulated after this diagram was drawn, somewhat altered the facts and percentages, but it was not deemed of sufficient importance to omit the observations, and there was no opportunity to alter the diagram. The characters are all given correctly to scale excepting that when one exists in less than 1 % of the specimens, it is represented by a line the thickness of which has no relation to the frequency of the same. On the left of the diagram is given the character a, all oculars exsert. This is an invariable feature in the specimens of Echinus affinis observed (p. 118). It is also the character of the OCULAR AND GENITAL PLATES. 151 adults of most Mesozoic and the young of all modern species of regular Echini. Passing to Echinus esculentus, all exsert is still strongly the character, but one plate may be insert as an infrequent progressive variant (p. 118). In Arbacia pundulata, a similar condition prevails, but one plate insert is a more frequent variant and rarely two or three plates are insert. Strongylocentrotus lividus has typically all oculars exsert, but one insert is a frequent and two insert a rarer progressive variant (p. 126).^ Echinometra mathaei has a lower percentage of all exsert and a consequent gain of progressive variants (p. 146). In Echinus angulosus the progressive variants become more strongly marked, especially as compared with the northern species of the genus shown (p. 119). In Salenocidaris varispina, instead of all exsert, one ocular insert is the species character, and the feature of oculars all exsert passes into the phase of an arrested variant (p. 112). Echinus magellanicus has typically one ocular insert; all oculars are exsert as an occasional arrested variant, or I, V insert as an occasional progressive variant. In no case seen were oculars I, V, IV insert, the space for which is indicated in the diagram at X, but in one speci- men oculars I, V, IV, II are insert, as an extreme progressive variant (p. 119). Echinometra lucunter from the West Indian fauna (p. 146) has one ocular insert as the species character, but all exsert is a frequent arrested variant and two insert a more frequent progressive variant, with rarely three plates insert. In the same species from Bermuda (p. 146) a different condition prevails, as arrested variants are rarer and progressive variants are much commoner than in the southern form. With Strongylocentrotus fragilis two oculars insert becomes the dominant character, with all exsert or one insert as frequent arrested variants (p. 128). In Echinometra van brunti two plates insert is strongly the dominant character with all oculars exsert or one insert as rela- tively infrequent arrested variants; and three oculars insert as a somewhat rare progressive variant (p. 147). Strongylocentrotus drobachiensis from York, Maine, has very strongly two oculars insert, with but few arrested and progressive variants (p. 138). From here on, the species shown have less of the bivium insert, few arrested variants with all exsert or one insert, and progressively more with three or four plates insert. Toxopneustes variegatus shows a distinct gain of progressive variants over the last species considered, and a consequent dropping of the bivium insert character (p. 121). In Strongylocentrotus purpuratus there is a still further reduction of the I, V insert and gain of the I, V, IV insert. A striking case is the Toxo- pneustes atlanticus in which progressive variants are much in excess of what is found in the closely allied T. variegatus (p. 122). With Tripneustes esculentus from the West Indian fauna (p. 124), three oculars insert becomes the dominant character. In this species from this area two oculars insert is a very ' In the diagram (text-fig. 176) the first 170 specimens only of Siroiigylocentrotus lividus are included. Recently, and too late to alter the diagram, this number was increa.sed to 1,163 specimens. In this larger series the percentages (pp. 126, 162) are altered somewhat from that shown in the diagram and one specimen, 0.1% has oculars I, V, IV insert, a feature not given in this diagram. 152 ROBERT TRACY JACKSON ON ECHINI. common arrested variant in frequency closely approaching the dominant character, and four oculars insert is a common progressive variant; rarely all oculars are insert. Arbacia nigra has strongly three oculars insert with no progressive variants, but occasional arrested variants of all exsert, and frequent arrested variants with one or two oculars insert (p. 116). This species covers the same range of characters as shown in Arbacia punctulata, but in very different pro- portions. Centrechimis setosus has strongly three oculars insert as the species feature and is almost the only species in the order in which the variation covers six characters. All exsert or one or two oculars insert occur as rare arrested variants, and four oculars or all insert are frequent progressive variants (p. 108). Of Centrostephanus rodgersi there are few observations, but as far as they may be trusted, it is the strongest three oculars insert of any species known, with frequent progressive variants having four or all oculars insert (p. 110). Eiicidaris tribu- loides has strongly three oculars insert as a dominant character (p. 97). Its range of varia- tion covers practically all the characters known in regular Echini from the Lower Carboniferous up. The only exceptions to this statement are cases of Echini that typically have an aberrant ocular arrangement, as Gymnechinus pulchellus (p. 120) and Strongylocentrotus gibbosus (p. 145). The arrested variants of Eucidaris tribuloides are few, with all oculars exsert, 'or one or two insert. Progressive variants are common with four or much oftener five oculars insert. In no sea-urchin studied is four oculars insert a typical species character, though this is said (Cotteau, 1875-'80) to be the case in Acrosalenia pseudodecorata Cotteau. With Phyllacanthus annulifera all oculars insert is the character with three or four oculars insert as frequent arrested variants (p. 102). In Phyllacanthus baculosa all oculars are insert as a strong species feature (p. 102). The only possible variants on this line of development are arrested, and such occur with three or four plates insert as rare arrested variants. In Aspido- diadema nicobaricum all oculars are strongly insert and no variation is known (p. 104). Those cases given in the diagram are selected, but almost any of the species shown in the tables of ocular plate arrangement could be intercalated in the diagram and there find an harmonious place in the scheme of differential structure. The accompanying tables show the characters of ocular plates in the species studied. The table for the Cidaroida is on pp. 100, 101 and for Strongylocentrotus drobachiensis on pp. 142, 143. In tabulating characters the only specimens omitted were those that had no locality, where this was an important item; also 71 that had three, four, or six areas, and some eight speci- mens that were so distorted that they could not properly be included. The general relation of percentages is considered on p. 93. All aberrant variations in the Centrechinoida are given in the last table (p. 164). As the relation of ocular plates to the genitals and periproct plaj's such a distinct and definite part in geological sequence, in ontogeny, in variation, and in geographical distribution, it seems that they may be accepted as a feature of importance in the study of Echini. It seems also that they make a reasonable basis where differences occur on which to arrange the systematic sequence of species within the genus. OCULAR AND GENITAL PLATES. 153 V/////^////Y////Y/^///^^////>y//// X/^Azy/ //// >//////^//////. 605 Echinus aiRnis 20(1 Et'hinus csculcntus 2,100 Arbacia punctulata, Woods Hole 170 Strongylocentrotus lividus 159 Echinomctra inathaci 100 Kchimis angulosus 2(3 Salcnocidaris varispina 1(J4 Echinus niagcUanicus .■(78 Ecliinomctra lueuntcr, West Indies and Florida 17.5 Ecliinomctra lucunter, Bermuda 5.5 Strongylocentrotus fragilis 76 Echinomctra van brunti 2,700 Strongylocentrotus drobach- iensis, York, Maine 1,013 Toxopueustes variegatus 120 Strongylocentrotus purpuratus 1,463 Toxopneustes atlanlicus, Bermuda (45-60 mm. diam.). 455 Tripneustes esculentus, West Indies and Florida 246 Arbacia nigra 1,168 Ccntrechinus setosus 12 Ccntrostcphanus rodgersi 849 Eucidaris tribuloides 12 Phj'llacanthus annulifera 106 Phyllacanthus baculosa 47 Aspidodiadema nicobaricum 154 ROBERT TRACY JACKSON ON ECHINI. Table of Typical Ocular Plate Arrangement and Variation in the Centrechinoida. 00 a a 1 a 3 Order CENTRECHINOIDA. Suborder A UWDONTA . £ 3 O < i a f— ( ►— 1 1— t l-H > > o GO .s i 3 o tf > a > 2 3 o > > _C3 3 o O Xt >< q; t-> 1— < > ►— 1 CO 3 o 1 X if .5 > > I— T 2 _d "s o o t4 c 2 "3 o o 5 a -i 2 a 3 u o ««-■ o a S O i Hemicidaridae. luu* 15* 100 6 S9 151 81 47 13 2 // 2 90 9 100 16 100 43 100 75 6 Jurassic, Europe. Heinicidaris hoffmanni Agassiz. Jurassic, Europe. Hemicidaris intermedia (Fleming). Jurassic, Europe. (Text-fig. 75.) Hemicidaris crenularis (Lamarck). Jurassic, Europe. Hemicidaris ludensis d'Orbigny. Jurassic, Europe. (Text-figs. 76, 77.) Hemicidaris langrunensis Cotteau. Jurassic, Europe. Hemidiadcma stramonium Agassiz. Jurassic, Europe. Asterocidaris minor Cotteau. Jurassic, Sollies, France. (Text-fig. 78.) Goniopygus peltaiits Agassiz. Jurassic, Europe. Glyplicus hierogbjphicus Agassiz. Jurassic, Europe. (Text-fig. 79.) 169 8 13 16 9 31 5 T 10 1 3 5 1 5e 9 67 12 58 ;2 16 1 18 10 16 43 1 75 426 ■ 366 32 27 1 Aspidodiadematidae. 44 AnTtitlndiadp.ma mriierei. Dodorloin. lOU 44 47 /OO 10 m 22 47 Hawaiian Islands, 241-294 fathoms. Aspidodiadema nicobaricum Doderlein. Hawaiian Islands, 165-500 fathoms. (Text-fig. 176.) Dermalodiadema antillarum A. Agassiz. West Indies, 687-955 fathoms. (Text-fig. 80.) Dermalodiadema horridum A. Agassiz. Galapagos Islands, 812 fathoms. 10 22 123 123 * Italic numerals represent percentages, Arabic numerals the number of specimens observed. OCULAR AND GENITAL PLATES. 155 Table of Typical Ocular Plate Arrangement and Variation in the Centrechinoida (continued). ■e 1 ■g -M (H ■4^ m OJ OJ txi ^ CO W X >1 y. — ' 1 I— 1 t« t—i t—i 1— 1 ! o hH h^ (— < »-H 1— ( ^ ^ t—i •-^ t—t HH *~* (—1 1 £2 • > > > tn CO M cj HH HH »-H Oj ^ 5 M o a > 1—1 Eh .S 1— 1 O *s Lh V t-l S > ►> ;-• "o 5S i S c 1— 1 to -t-3 en V CO .s .s > > >■ C OJ "o 1 t— ( > 1— r 1— T 1 H-T a c3 m m CO CD CO U] r^ Lh t4 (h t-< 3 •S Cj _S -2 ci ^ c3 o Q a a ^2; o "3 "3 "a "3 •3. o c! Suborder A ULU DON T A (contiiuRd). < o O o O O O < ^ Centrechinidae. 18 Pseudodimkma pseudodiadema (Lamarck). Jurassic, Trouvillc; Yonnc. (Text-fig.s. 81, 82.) 89* 11 16* 2 1 Magnosia punciala Quenstedt. Jurassic, Nattheim. (Text-fig. 83.) loo' 1 3 Cotlaldia granulosa (Munster). Cretaceous, Chamboy. 100 3 4 Phymechimis mirabilis (Agassiz). Jurassic, Europe. (Text-fig. 84.) 75 ;g5 S 1 7 Pediiia rolala Wright. Oolite, England. 100 7 4 Stomechinus granularis Agassiz. Jurassic, Europe. 100 4 28 Slo7>iechinus perlalus Desnioulins. Jurassic, Europe. 100 28 50 Stomechinus bigrauularis (Laniarcli). 88 2 2 8 Jurassic, Europe. (Text-figs. 85, 86.) 44 1 1 4 14 Polycyphus tiormannus Desor. Oolite, Europe. 93 7 13 1 37 Orthopsis niiliaris (d'Archiac). Cretaceous, Algiers; Europe. (Text-fig. 87.) 100 37 ' Cenlrechinus sctosus (Leske). Atlantic and Pacific Oceans, several localities. De- 23 5 9 55 5 3 ;2 veloping series, 8-40 mm. diam. (110 specimens). 25 5 10 60 5 3 2 1,278 (Text-figs. 88-92.) The same. Atlantic and Pacific Oceans, many localities. Devel- 0.2 0.2 0.; 1 57 /7 ^;2 S oped series, 40-108 mm. diam. (1,168 specimens). 2 2 1 ii 671 194 254 33 ■ (Text-figs. 93-95, 176.) 11 Echinoihrix calamaris (Leske). 5 91 Mauritius; Zanzibar; and loc. (?) 1 10 18 Echinoihrix diadema (Linnd). Mauritius; Hawaii; Bonin; Samoa. ;oo 18 ' Italic numerals represent percentages, Arabic numerals the number of specimens observed. 156 ROBERT TRACY JACKSON ON ECHINI. Table of Typical Ocular Plate Arrangement and Variation in the Centrechinoida (continued). i g "o u Xi a 3 Suborder A I/LODOATTA (continued). X 2 "3 o -H H-l h-T 1— 1 > > .s 1— ( "3 O O! I-H (— ( h- T 1— 1 > 1— T if .S > 2 o o ID hH hH ►-H h- T > 1— 1 tf '/.■ .S > o a> s§ 1— 1 l-H HH 1— 1 +3 > § o 1 1— 1 l-H 1— t ■*^ .s h— 1 HH k> »— ( > W "3 O o .s 'i O < O -i O S a > 3 o 5 1 69 11 100 2 70 7 73 19 81^ 13 > 65 40 2 92 93 se 50 6J 10 5S 7 162 19 3 * Italic numerals represent percentages, Arabic numerals the number of specimens observed. 158 ROBERT TRACY JACKSON ON ECHINI. Table of Typical Ocular Plate Arrangement and Variation in the Centrechinoida (continued). E m 1 Suborder Sr/iJOZ)OA^rA (continued). £ O < 1-^ I— ( > > 1 a CO — o > 1— I 1— T .s > "3 O >> )— * % a > 1— 1 O ti O) tc tf 'fl a > > 1— T £ O 1 hH HH 1— I 1 I— I I— I > l-H > £ 3 o O 1 3 o 3 o o "o § bo c 10 Phyiuosomatidae . Phyinusonui dilii/imrei (Desor). Cretaceous, Algiers. (Text-fig. 106.) 100* 10* 10 Stomopneustidae . Stoinopneustes varluluris (Lamarck). Ceylon; Mauritius; Zanzibar. (Text-figs. 107-110.) 10 04 2 1 70 45 22 14 6 4 64 Arbaciidae. Arbacia sldlata Gray. Gulf of California; Panama. 1 45 14 4 78 95 74 H 132 87 1,832 76 174 87 58 9 7 1 3 100 2 0.1 2 3 8 7i 238 19 43 -4 3 30 23 2;? 55 1 141 ; lediterranean; Azores; Teneriffe. Arbacia punclulata (Lamarck). Woods Hole, Massachusetts (2,100 specimens). (Text-figs. 111-114, 200-205, 176.) The same. . Florida (229 specimens) . rhnn'yt. iliifrpfmi. Rlninvillp O.S 7 4 / 1 11 12 30 OJt 9 2 1 0.6 2,329 12 3 67 6 7 76 Cliili; Patagonia; Hawaiian Islands (?) Arbacia spatxdigera (Agassiz). Peru. Arbacia nigra (Molina). ChiU. (Text-figs. 167, 176.) Coelopleurus floridanus A. Agassiz. Cape Hatteras to Charleston, 63 fathoms. 53 25 57 139 5 13 246 10 2 19 2,939 2,282 2 373 53 176 53 * Italic uuiuerals represent percentages, Arabic numerals the number of specimens observed. OCULAR AND GENITAL PLATES. 159 Table of Typical Ocular Plate Arrangement and Variation in the Centrechinoida (continued). QQ g 1 & "o 1 Suborder CAMARODONTA. "3 O 1— 1 H-1 > > f ! .£ ! »— t ■3 Oculars V insert; I, IV, II, III exsert. t t— 1 > 1— 1 c « 1^ 1 % ! dJ 1 1— 1 1— 1 C > 1— T ■3 •2 i t— 1 1—1 > 1— T 2 1 a; 'n a 'Tl "3 -Q a a; i c 7 Temnopleuridae. Glyphucyphus radialus Desor. Cretaceous, Europe. Trigonocidaris alhida A. Agassiz. West Indies. Temnopleunis hardwickii (Gray). Japan; Ceylon. Temnopleurus toreumaticus (Leske). China Seas; Japan; India. Temnopleurus reevesii Gray. Japan; Siam. Salmacis alexandri Bell. Australia. Salmaris bicolor Agassiz. Japan; Mauritius; Red Sea. Salmacis sphaeroides (LinnS). Australia; Philippines; and loc. (?) Mespilia globula (Linn6). Samoa; Japan; and loc. (?) Microcyphiis maculalus Agassiz. Mauritius; Japan, etc. Amblypneustes formosus Valentin. Australia; Tasmania. Amblypneustes griseus (Blainville). Australia. Amblypneustes pallidus (Lamarck). Australia. Amblypneustes ovum (Lamarck). Australia. Holopneusles purpurescens A. Agassiz. Australia. 7* 11 100 11 100 7 100 34 63 10 100 22 100 14 100 12 97 33 9S 136 100 100 12 100 10 97 37 97 32 7 34 19 47 9 22 14 12 34 3 1 139 1 1 g 2 12 10 38 3 1 33 3 1 400 378 11 1 1 7 2 ' Italic numerals represent percentages, Arabic numerals the number of specimens observed. 160 ROBERT TRACY JACKSON OX ECHINI. Table of Typical Ocular Plate Arrangement and Variation in the Centrechinoida (continued). B Suborder CAM A RODONTA (continued). o > > > O .s > i 605 10 200 129 299 10 127 100 200 16 14 Echinidae. Echinus affinis Mortensen. Off east coast of United States. 1,022-1,106 fathoms. (Text-figs. 115, 176.) Echinus elegans Diiben and Koien. Off Cape Remain and loc. (?) Echinus gracilis A. Agassiz. Off Carolina and Martha's Vineyard. Echinus esculentus Linne. Plymouth, England; Isle of Man; Mediterranean. (Text-figs. 116, 117, 176.) Echinus miliaris Milller. Norway; England; Isle of Man. Echinus microluberculalus Blainville. Mediterranean. Echinus melo Lamarck. Mediterranean; England. Echinus acutus Lamarck. Plymouth, England; Norway; Bergen; Mediterranean. Echinus angulosus Leske. Algoa Bay; Cape Colony; South Africa. (Text-fig. 176.) Echinus magellanicus Philippi. Patagonia; Straits of Magellan. Developing series, 2.5-5 mm. diam. (36 specimens). (Plate 3, fig. 14.) The same. Falkland Islands; Straits of Magellan. Developed series (164 specimens). (Text-figs. 165, 176.) Echinus margaritaceus Lamarck. Patagonia; Kerguelen Islands; Antarctic. Gymnechinus robillardi (Loriol). Mauritius. (Text-fig. 179.) Gymnechinus pulcheUus Mortensen. Singapore; Gulf of Siam. (Text-figs. 177, 178.) 100 * 605* 1_(J0 8 100 10 97 195 9S 126 96 287 90 9 83 104 54 54 97 35 5 9 6 1 2 5 W 1 IS 23 SO 30 145 6 1 n 1 3 2 0.3 1 4 7 56 OM 1 e_ 1 1 2 6 1 S9 8 100 14 * Italic numerals represent percentages, Arabic numerals the number of specimens observed. OCULAR AND GENITAL PLATES. 161 Table of Typical Ocular Plate Arrangement and Variation in the Centrechinoida (continued). Eh 1 1 X CD 1 1 4) -*-3 l-H ^ l~J »— I c .a K o 1 Suborder CAMARODONTA (continue!). CO a o < > > CO _c l-H o t— t > 1— I a* > O o > -S 1 O Vj > > "a O CD en ^B HH > "3 o o (-1 GJ i 1 < •i s OS g o "3 a 60 o 2 < Echinidae (continued). 5 Toxop/ieimles maculalus (Lani.arek). Fiji Island.s; Mauritius. 20* 1* 50 4 38 Tnxopneusles pileolus (Lamarck). 8 a^' Acapulco; Panama; Mauritius. 3 35 25 Toxopneusk'S semUuherculalus Valentin. Galapagos Islands. 1 5e 24 1,043 Toxopneustes variegatus (Lamarck). 1 0.^ 90 5 0.^ 0.8 West Indies and Florida. (Text-figs. 170, 184.) 11 1 937 84 2 8 cToxopneusles allanlicus (A. Agassiz). 0.9 0.2 5; i6 o.s 2 Bermuda. Developing series, 35-45 mm. diam. 5 1 475 92 2 12 (587 specimens). The same. 0.1 0.5 0.; 67- 28 .f.g 2.5 2,643 ■ Developed series, 45-60 mm. diam. (1,403 speci- mens). (Text-figs. 118-122, 176.) 1 8 2 983 415 18 36 The same. 0.7 0..3 68 ;g9 0.7 1.8 . Developed series, 60-77 mm. diam. (593 specimens). 4 2 401 171 4 11 16 Tripneusles depressus A. Agassiz. 6 7.5 19 La Paz, California; and loc. (?) 1 12 3 72 Tripneusles variegatus (Leske). 7 ; 6S 5 11 4 Mauritius; Jeddah, Red Sea; Durb.an; Natal; Samoa; 5 1 49 6 8 3 Indo-Pacific; Lord Howe Island; H.awaiian Islands. Tripneusles esculenlus (Leske). 2 (J7 35 2 0.5 1 Bermuda. Developed series, 50-145 mm. di.am. (193 3 117 67 3 1 o specimens). (Text-fig.s. 123, 196.) The same. West Indies and Florida. Developing series, 19-50 .9 5 76 42 13 7 ^> 703 1 mm. diam. (55 specimens). The sa}ne. West Indies and Florida. Developed series, 50-127 2 3G 165 172 ^5 84 5 ff 27 mm. diam. (455 specimens). (Text-figs. 124-127, I 176.) 20 Ei'echinus chlorolicus (Valentin). New Zealand. 90 18 5 1 5 1 6,292 1,444 259 19 3,290 1,021 123 7 129 * Italic numerals repi-esent percentages, Arabic numerals the number of specimens observed. 162 ROBERT TRACY JACKSON ON ECHINI. Table of Typical Ocular Plate Arrangement and Variation in the Centrechinoida (continued). p. a 3 10 48 291 1,163 60 55 Suborder CAMARODONTA (continued). Slrongylocenlrolus alhu Patagonia. 39 12 56 179 120 24 2,063 Strongylocentrotidae. Pseudoboletia indiana (Michelin). Mauritius; Isle de la Reunion. Sphaerechinus ptdcherrimus A. Agassiz. Japan. Sphaerechimis granulans (Lamarck). Mediterranean; Azores. Stro7igylocentrotiis liiridus (Lamarck). Mediterranean; Fayal; England. (Text-figs. 128-130, 176.) (Molina). Strongylocentrotus fragilis sp. nov. Catalina Islands, California; off California and Ore- gon coasts. (Text-fig. 176.) Slrongylocenlrolus mexicanus A. Agassiz. Cape St. Lucas. Slrongylocenlrolus tuberculalus (Lamarck). Japan; Sidney, Australia. Slrongylocenlrolus depressus A. Agassiz. Japan and loc. (?) Slrongylocenlrolus drbhachiensis (O. F. Miillcr). See tabulation of 27, 417 specimens exclusi\'e of devel- oping series, p. 143. The percentages only are given here. (Text-figs. 135-149.) Slrongylocenlrolus eurylhrogrammus (Valentin). Australia. Slrongylocenlrolus franciscanus A. Agassiz. California; Washington. (Text-figs. 151-153, 106.) Slrongylocenlrolus purpuralus (Stimpson). California; Puget Sound. (Text-fig. 176.) Slrongylocenlrolus gibbosus (Valentin). Payta, Peru. (Text-figs. 154-157.) 876 53 32 13 0.004 03 "3 O 1 2 IL 195 2 1 29 16 915 220 26 S 3 17 1 > C3 "3 O 0.04 I 100* 10* 98 47 59 258 50 15 56 31 S3 5 92 36 100 12 94 93 52 S7 156 77 93 31 765 c > 3 o > > o o 3 1 3 n 20 22 26 i 1 56 0.03 17 4 C3 9^ 25 15 IS 1.2 79 19 69 * Italic numerals represent percentages, Arabic numerals the number of specimens observed. OCULAR AND GENITAL PLATES. 163 Table of Typical Ocular Plate Arrangement and Variation in the Centrechinoida (continued). 03 § - S P. s 3 2; Suborder CAMARODONTA (continued). 03 u o 1 HH > > a> CO -S t— 1 t-, 3 o H- 1 t—t 1— 1 f > o O t-i 1— 1 1— 1 > C ;-< C3 o 1 h- ( i > o o 1 cJ h- 1 t— 1 H-t .^ .9 1— < > > "3 o o CO G 1 o o < a o3 b« -i Mi t~i "3 o o "o c a> S 0) bD G t-i 44 Echinometridae . Echinontdra ohlonga Blainville. Samoa; Hawaiian Islands; South Africa. Echinometra viridis A. Agassi z. West Indies. Echinometra malhaei Blainville. Red Sea; Durban; Natal; Mauritius; Bonin; Hawai- ian Islands. (Text-fig. 176.) Echinometra lucunter (Linn6). West Indies and Florida (578 specimens). (Text- ■ figs. 158-161, 176.) The same. Bermuda (176 specimens). (Text-fig. 176.) Echinometra van brunti A. Agassiz. Lower California; La Union; South America. (Text- fig. 176.) Heterocentrotus mammillatus (Leske). Mauritius; Indian Ocean; Bonin Island; and loc. (?) Heterocentrotus trigonarius (Lamarck). Hawaiian Islands; Mauritius; Tahiti; Palmyra Is- land. Colobocentrotus atralus (Linne). Hawaiian Islands; Mauritius; Zanzibar; and loc. (?) 98* 43* 96 24 79 126 IT 97 9 15 // 8 80 28 GO 28 99 81 o.s 2 / 2 4 3 S 1 4 2 1 / 1 19 31 57 332 46 81 /^ 9 // 4 ;25 13 1 25 159 1 25 144 76 72 55 S 1 5 4 1 754 o.s 2 0.ff 1 ; 1 1 1 0.6 76 1 35 3 47 1 82 1,222 450 10 473 281 4 4 ' Italic inunerals represent percentages, Arabic munerals the number of specimens observed. 164 ROBERT TRACY JACKSON ON ECHINI. Table of Aberrant Ocular Plate Arrangement in 4S,541 Specimens of the Centrechinoida} DO 1 1 "S B 3 en g 1 a m V HH ^H > 1— ( 1 > u ■i b. ! S! £ > > .£ t— < ^—^ m s u O % X > fi a 3 O o % X > bl en .£ > o 0) 1—4 > > b< _c )— 1 2 "3 O 1—1 I— ( > »— » > i" .s 1— t 1—1 2 ta "3 £ 1— ( t— ( »— 1 )— c > > CO (-< 1 o 4^ (-> X > bi 0) en -S 1— 1 t>" t s "3 o O t-i > 1— < 1—1 > tn 1 <5 en > tn C > 1— 1 tn t> _S 3 o 1 ID 1— 1 > )— 1 (-> _c > m t^ _2 "3 o O b> X > > JS "B tj O •4-J bi X t- OJ .s > > CO "3 o OJ 0^ 1— 1 > > cfi *3 O o t-, OJ 4; tn C > O 58 Hemicidaris crenularis (Lamarck). Centrechiniis selosus (Leske). Acrosalenia heniicidaroides Wright. Acrosalenia pustulata Forbe.s. Shmopneustes variolaris (Lamarck). Arbacia lixula (Linne). Arbacia punclulata (Lamarck). Arbacia dufresni Blainville. Arbacia spatuligera Agassiz. Arbacia nigra (Molina). Microcyphus maculalus Agassiz. Echinus microluberculatus Blainville. Echinus angulosus Leske. Echitius magellanicus Philippi. Echinus margarilaceus Lamarck. Gymnechinus robillardi (Loriol). Gymnechinus ptdchellus Mortensen. Toiopneusles variegatus (Lamarck). Toxopneusles atlanticus (A. Agassiz). Tripneusles variegatus (Leske). Tripneusles esculentus (Leske). Evechinus chloroticus (Valentin). Sphaerechinus granulans (Lamarck). Strongytocentrotus Hindus (Lamarck). Strongylocentrotus albus (Molina). Slrougyloceidrolus luberculalus (Lamarck). Strong yloceidrotus drobachiensis (0. F. Miiller) Strongylocentrotus franeiscaniis A. Agassiz. Strongylocenlrolus gibbosus (Valentin). Echinometra mathaei Blainville. Echinometra lucunler (Linn6). HeleroccHlrolus mammillalus (Leske). 2 3 7 6 6 1 0.6 7 13 8 1 1 1 1 6 89 100 0.8 2 4 4 5 9 1 13 3 1.2 1 79 1 0.3 3 1 1 278 1 1 1 32 58 1 1 2 16 64 1 1 2 2 3 141 . . 2 329 1 1 14 3 10 13 67 76 246 1 1 4 139 1 299 2 1 100 200 1 16 1 9 8 14 14 1,043 2 643 2 1 1 18 1 1 5 25 13 1 72 2 703 1 1 20 2 1 20 291 2 23 1,163 5 4 2 3 60 4 2 1 39 1 81 33,000 179 3 2 29 33 34 1 215 1 2 •> 24 5 8 159 1 1 1 63 754 1 35 3 8 4 300 1 11 17 Total number of aberrants 704 = 10 97 1 56 82 4 47 'This table includes all aberrant variants of the Centrechinoida listed in the tables, pp. 142, 143, 154 to 163. See text- figs. 110, p. 114; 140-149, p. 134; 155, 156, p. 145; 177-179, p. 165; 182, p. 165; 196, p. 169. SPECIAL CHARACTERS OF GENITAL PLATES. 165 Special Characters of Genital Plates. Certain characters of genital plates are here taken up aside from theii- relations to the oculars, that have just been considered. The characters in genitals are subject to a great deal of variation. While the bearing of these variations is often obscure, for the most part they follow very definite lines. In my studies of post-Palaeozoic fossil and Recent Echini, over Text-figs. 177-184. — Showing genital plates excluded from the periproct. 177. Gymrwchinus pulchellus Mortensen. Gulf of Siam. Diam. 15 mm. R. T. J. Coll., 863. X 8.2. Genital 3 exsert by the contact of genitals 2 and 4. Oculars I, II insert, the species character (pp. 93, 120). 178. The same. Diam. 10 mm. R. T. J. Coll., 864. X 8.7. Genital 4 ex.sert by the contact of genitals 3 and 5. 179. Gymnechinus robillardi {Lonoi). Mauritius? Diam. 26 mm. R. T. J. Coll., 811. X 4.5. Genital 3 exsert. Oculars I, II insert, the species character (p. 120). 180. Strongylocenlrotus drobachierisis {O.F.MulleT). YorkHaiboT, Maine. Diam. 44 mm. R. T. J. Coll., 782. X 2.81. Genital 3 exsert by the contact of genitals 2 and 4, as in text-figs. 177, 179, 184. 181. The same. Chelsea Beach, Massachusetts. Diam. 3 mm. R. T. J. Coll., 880. X 28. Genital 4 exsert by the contact of genitals 3 and 5 as in text-fig. 178. No genital pores (compare text-fig. 133, p. 129). 182. The same. Dumpling Island, North Haven, Maine. Diam. 50 mm. R. T. J. Coll., 866. X 2.7. Genital 4 exsert by the contact of genital 5 and ocular IV (compare text-fig. 144, p. 134). 183. The same. Calderwood Island, Maine. Diam. 32 mm. R. T. J. Coll., 865. X 2. 8. Genital 4 exsert by the contact of genital 3 and ocular V. 184. Toxopneastes varicgatus (Lamarck). Boca Ceiga Bay, Florida. Diam. 58 mm, R. T, J, Coll., 872. X 3.7, Genital 3 exsert (p. 121). 166 ROBERT TRACY JACKSON ON ECHINI. 250,000 genital plates have been examined, and the principal striking variations noted. In the 33,000 specimens of Strongylocentrotus drobachiensis studied, detailed lists of the variations of genital plates were kept as a basis for determining the numerical frequency of occurrence. Under ocular plates it is shown that there is great diversity as to whether these meet the periproct or are shut out from it. On the other hand, genital plates show great uniformity in that all the plates meet the periproct. No exception to this is known in fossil regular Echini; in Recent Echini, however, some exceptions have been found as individual variations. Dr. Mortensen (1904, p. 114) says of Gymnechinus pulchellus that sometimes one of the genitals, mostly 3, is excluded from the periproct. He writes me that in about thirty percent of the specimens genital 3 and more rarely 4 is exsert. In ten selected specimens of G. pulchellus that Dr. Mortensen generously sent me, four have genital 3 exsert (text-fig. 177), two have genital 4 exsert (text-fig. 178), and four have all genital plates reaching the periproct. In a specimen of Gymnechinus robillardi (text-fig. 179) genital 3 is exsert. De Loriol (1883) figures the same species with genital 4 exsert. In his collection I saw two specimens with genital 4 exsert, as in text-fig. 178. It is striking that this very exceptional character is common in two related species. Specimens with exsert genitals have been found in a few other species. One specimen in 139 Microcyphus maculaius (this is in de Loriol's collection in Geneva), one in 1,043 Toxopneustes varicgatus (text-fig. 184), and two in 2,643 Toxopneusies atlanticus have genital 3 exsert. In 33,000 Strong ylocentrotus drobachiensis 37 specimens have genital 3 exsert (text- fig. 180). In all the cases seen of genital 3 exsert in the several species, this plate was similarly excluded by the contact of genitals 2 and 4 as figured. In S. drobachiensis eight specimens have genital 4 exsert. This, however, is not excluded by the same plates in all cases. In a very young individual (text-fig. 181) genital 4 is excluded by the contact of genitals 3 and 5, as in Gymnechinus pulchellus (text-fig. 178). Another method of exclusion of genital 4 is by the contact of genital 5 and ocular IV (text-fig. 182) ; and in a third method, which may be considered more normal for the species, genital 4 is excluded by the contact of ocular V and genital 3 (text-fig. 183). In the Museum of Comparative Zoology, a Microcyphus maculatus from Mauritius has genital 4 excluded by the contact of ocular V and genital 3, as in text-fig. 183; also a Heterocentrotus trigonarius from the same locality has genital 3 excluded by the contact of genitals 2 and 4, as in text-fig. 184; and a Microcyphus annulatus Mortensen, from Bass Straits, Australia, has genital 1 excluded from the periproct by the contact of genitals 5 and 2. The oculars of this specimen are all exsert, as usual in the genus. In all, 61 cases have been seen in 50,000 specimens with a genital excluded from the peri- proct, 47 of the cases being of genital 3, thirteen of genital 4, and one case of genital 1. Why genital 3 or 4 should be excluded and almost no others is not obvious, Init it shows how definite even a very rare variation can be. While genitals are rarely excluded from the periproct as variants, they are still more con- SPECIAL CHARACTERS OF GENITAL PLATES. 167 stantly in contact with the interambulacrum ventrally. In Bothriocidaris archaica (Plate 1, fig. 2) the genitals Ue wholly dorsal to the oculars, and in B. pahleni (Plate I, fig. 6) they do so in part. In one specimen of Strongylocentrotus drdbachiensis (Plate 5, fig. 16) genital 1 lies dorsal to the oculars, as in Bothriocidaris archaica; but in this peculiar specimen interambula- crum 1 is wanting (p. 42), which accounts for this exceptional genital exclusion. Otherwise, as far as seen, a genital, when existent, always extends to the interambulacrum of its area. Genital plates are of secondary importance. One may be absent without visibly affecting the corona (spatangoids, text-figs. 174, 175, p. 149). Rarely a genital may be wanting in regular Echini (p. 45), when the other portions of the test have the usual pentamerous system. Such a case is seen in Eucidaris (text-fig. 185). In this specimen there are five oculars, I and II being in contact from the absence of genital 1. In interambulacrum 1 there are two young plates lying against oculars I and II ; the next older plate belonging to column 2 is large and extends across the area, whereas the third older plate is small. Otherwise the corona is quite as usual. In two specimens of Ai'bacia (Plate 4, figs. 11, 12) and in a Strongylocentrotus (Plate 6, fig. 7), genital 4 is want- ing and interambulacrum 4 is narrowed, consisting in part of only a single column of plates. In Tripneustes csculcntus (Plate 6, fig. 4) there are six oculars, but no additional genital, so that the extra genital may theoretically be considered as wanting, and there is only a single column of interambulacral plates through- out the added area as far as developed. In all of these specimens in the area where the genital is wanting, the interambulacrum dorsally abuts against the two over- lying ocular plates in a similar manner to that of Bothriocidaris and the posterior area in spatangoids (text-figs. 162, 174, 175 p. 149). Genital and ocular plates are fused in a mass in clypeastroids, and two genitals may be fused in spatangoids (A. Agassiz, 1904, p. 161), but in regular Echini these plates are typically separate. Occasionally, however, in regular Echini two genitals or a genital and an ocular are fused. In a specimen of Centrechinus setosus (text-fig. 186) genitals 2, 3 and ocular III are fused into a perfectly symmetrical mass, no trace of sutures being visible. A similar fusion of two genitals, or a genital and an ocular, has been seen in many cases in the Echinidac and Strongylocentrotidae. In Strongylocentrotus drdbachiensis genitals 2, 3 were fused in 36 cases; 3, 4 in 32 cases (text-fig. 195) ; 4, 5 in 68 cases (text-fig. 147, p. 134) ; and 5, 1 were fused in 19 cases (text-fig. 148, p. 134). Genitals 1, 2 were fused in only five cases. The fusion of genitals 185 Text-fig. 185. — Euciilaris Iribidoides (La- marck). Jamaica. Diam. 34 mm. R. T. J. Coll., 802. Genital 1 wanting. Young interam- bulacral plates originate against ocular.s I and II, as usual (p. 45). 168 ROBERT TRACY JACKSON ON ECHINI. was seen in other species also, as Tripneustes esculentus (text-fig. 196). In 103 cases of S. drobachiensis an ocular was fused with a genital (text-fig. 143, p. 134), any ocular of the five fusing with one or rarely two of the associated genitals. As far as the fusion of genitals, or genitals with an ocular goes, it may be considered a parallel variation to the typical condition in certain irregular Echini, where such fusion normally occurs. While fusion of genitals is rare, splitting on lines of solution to make divided plates is more common. This splitting of genitals by secondary sutures is a typical character in the Echino- thuriidae only (Asthenosoma, text-fig. 101, p. 109, and Phormosoma, text-fig. 170, p. 149). Such splitting is shown fully by Mr. Agassiz (1904) in many of this family. The splitting is t86 Text-fk:s. 1S6-1S9. — Fusion, or siilitting of genital i)Iatcs in Coilrechinus seiosus (Lcsko). 186. .Jamaica. Diam. 49 mm. R. T. J. Coll., 803. X 2.8. Genitals 2, 3 and ocular III arc fused in a mass; oculars I, V, IV, II insert, bilaterally symmetrical through III, 5. 187. Bahamas. Diam. 85 mm. R. T. J. Coll., 714. X 2.8. Genitals .spht by secondary sutures. 188. Bahamas. Diam. 85 mm. R. T. J. Coll., 689. X 2.8. Madreporic pores in genitals 2, 3, .and ocular III. 189. Jamaica. Diam. 71 mm. R. T. J. Coll., 772. X 2.8. Madreporite split by secondary sutures. especially marked on the ventral border of genital plates or may pass directly through the plate. I have seen no splitting of genitals in fossil Echini, but it is rather common as a variant in Recent species. Such secondary division of genitals is seen in Centreehinus (text-figs. 187, 189). In Sirongylocentrotus drobachiensis many cases of split genitals were found. In 275 cases genital 3 is spht by a transverse suture parallel to the base, as in text-fig. 190. This seems to be a family peculiarity, as it was observed in several species of the Echinidae as in Tripneustes esculentus (Plate 6, fig. 4), and in the Strongylocentrotidae. Instead of splitting by one trans- verse suture, as in text-fig. 190, genital 3 may be divided by one or more sutures in other planes, as in text-fig. 190a and 19t)b; such were noted in 90 cases. Genital 3 is split much more SPECIAL CHARACTERS OF GENITAL PLATES. 1G9 Text-figs. 190-196. — Splitting, fu.sion, etc., of genital and ocular plates, in iStrongyloccntrotu.s and Trii)iu'ustos (p. 28) . 190. Slrongyloccntrotus drdbachiensis (O. F. Mtillei^. York Harbor, Maine. Diam. 54 mm. R. T. J. Coll., 773. X 3. Genital 3 split by one horizontal suture (compare Plate 6, fig. 4). 190a. The same. Diam. 38 mm. R. T. J. Coll., 774. X 3. Genital 3 split by one vertical suture. 190b. The same. Diam. 49 mm. R. T. J. Coll., 775. X 3. Genital 3 split by three sutures. 191. The same. Calderwood Island, Maine. Diam. 46 mm. R. T. J. Coll., 879. X 3. Genital 2 spht. 192 sutures. 193 The same. Harpswell, Maine. Diam. 43 mm. R. T. J. Coll., 776. X 3. Several genitals split by secondary The same. York Harbor, Maine. Diam. 46 mm. R. T. J. Coll., 711. X 4. Genitals 2, 3, and oculars HI, V are split by secondary sutures; the splitting of oculars is a very rare variation (Plate 6, fig. 6). 194. The same. Diam. 42 mm. R. T. J. Coll., 700. X 4. Genitals 1 and 2 much split up; ocular V split; extra genital pores, but there is no genital pore in the madreporite (compare text-figs. 197- 199, p. 171). 195. Thesame. Diam. 41 mm. R.T.J. Coll., 777. X 3. Genitals3, 4fused (comparePlate6, fig. 2; Plate7, fig. 7). 196. Tripneustes esculenlus (Leske). Bermuda. Diam. 138 mm. R. T. J. Coll., 787. X 2. Genitals 5, 1 fused; oculars V, IV, II insert (pp. 93, 124). 170 ROBERT TRACY JACKSON ON ECHINI. frequently than an}^ other plate in the families of the Echinidae and Strongylocentrotidae. Genital 2 is also frequently divided in .S'. drobachiensis by secondary sutures (text-figs. 191- 194.) Divisions of this plate were found in 120 cases in this species. Genitals 1, 4, and 5 are occasionally divided by secondary sutures, as seen in 134 cases, but 2 and 3 take the lead by a strong majority. Very rarely oculars are split (text-figs. 193, 194), but only ten cases were noted; otherwise, genitals divided by secondary sutures are the only cases I have seen in Echini of plates derived by the fission of preexisting plates. In the young of echinothuriids, genitals are not split, and in Strongylocentrotus the splitting was very rare in young specimens, wliile rather frequent in adults. It is evident, as in echinothuriids, that this is a character that comes in with increasing age. Occasionally a line of secondary suture only goes partially through a plate, demonstrating that these are true cases of splitting and not cases of accessory plates. In very young Echini genital pores do not exist, as shown fully by Loven (1892) and A. Agassiz (1904). As seen in Strongylocentrotus drobachiensis (p. 131, text-figs. 131-134), no genital pores were found in specimens up to 5 mm. in diameter, after which age they come in rapidly, and only 128 in 800 were without such pores in the series 5 to 10 mm. diameter; only six in 2,000 were without genital pores in the series 10 to 15 mm. diameter, and no larger specimen was wholly imperforate. Occasionally, one, more rarely two, and in three cases three genitals were imperforate in adults. In a fresh Toxopneusies atlaniicus examined, in which a genital was imperforate, the corresponding genital gland was absent. The absence of a genital pore as a variant may be compared with Holectypus (text-fig. 171, p. 149), and some spatangoids (A. Agassiz, 1904) in which one or more genitals are typically imperforate. Typically, in post-Palaeozoic regular Echini there is a single genital pore within the con- fines of each genital plate. In Goniocidaris nutrix and canaliculata (text-figs. 72-74, p. 99) the ventral border of the genitals is open, doubtless resorbed so that the genital pores impinge on the interambulacra. This character has been seen as a variant in Centrechinus setosus, Arbacia nigra, and Strongylocentrotus drobachiensis, but is of uncommon occurrence. Mr. Agassiz (1873) shows that in the clypeastroid Peronella peronii (Agassiz) the genital pores lie in the interambulacra widely removed from their usual position. In an Arbacia punctulata in one area the genital pore is not in its usual plate, but passes through the third interambula- cral plate from the apical disc. In Strongylocentrotus the genital pores very rarely perforate an interambulacral plate, and in four cases found perforated an ocular plate. While genital pores are usually found in genital plates, it seems that there is no necessary correlation, and the pore when developed may perforate any portion of the test. Genital pores are usually perfectly visible in dorsal view, are even salient features, but in Salenia pattersoni they are invisible externally though plainly seen in an internal view of the same specimen (Plate 4, figs. 1,2). Wliile a single genital pore to a plate is the character in Recent regular Echini, supplementary pores are not rare. They are especially common in Arbacia SPECIAL CHARACTERS OF GENITi^L PLATES. 171 punclulnla (text-figs. 197, 198), in which I previously described them (Jackson, 1899, p. 130). Simikir extra genital pores have been seen in Eucidaris tribuloides, Echinus affinis (text-fig. 115), Strongylocentrotus franciscanus (text-fig. 199), ;S'. drohachiensis (text-fig. 193), and many other species. In S. drobachiensis extra pores were seen in 186 specimens. Dr. Mortensen writes me that such extra pores are connected with the genital glands. They are always sporadic Text-figs. 197-199.— Extra pores in genital plates. 197. Arbacia pimclulala (Lamarck). Woods Hole, Massachusetts. Diani. '6-1 mm. oculars exsert. 198. The .same. Diam. 35 mm. R. T. J. Coll., 655. X 3. Ocular V insert (p. 115). 199. SlrongyloceiUrolus franciscanus A. Agassiz. California. Diam. 123 mm. 11. T. J. Coll., 72(j. X 2. V insert (compare text-fig. 115, p. 117). R. T. J. Coll., 656. X3. All Oculars I, and a]:)parently may be considered a parallel variation to, rather than a genetic connection with, the Palaeozoic types where extra pores typically occur. In the Ordovician Bothriocidaris (Plate 1, fig. 2) genital pores are unknown; it may be that they were wanting, as such pores are wanting in young Echini. More likely they existed but do not show in external view, as noted in Salcnia pattersoni (p. 112). In an ancient fossil it would be easy for small genital and madreporic pores to be so filled up as not to be recognizable, for frequently in fossils pores cannot be seen when we know from other specimens that such existed. In Lepidechinus (Plate 63, figs. 7, 8) there is one pore in each genital plate in the only species of the genus in which the apical disc is known. In other Palaeozoic Echini genital plates typically have more than one pore to a plate. There may be two or three, as in Lepidesthes (Plate 68, fig. 5), or there may be three to five in a plate, as in Palaeechinus (Plate 29, fig. 6; Plate 30, fig. 4), Lovenechinus (Plate 41, fig. 3), and Melonechinus (Plate 56, fig. 6). Instead of a few pores there may be numerous genital pores to a plate, even as many as ten or eleven, as shown in Lepidocentrus (Plate 21, fig. 5), Pholidechinus (Plate 28, fig. 10), and Perischocidaris (Plate 67, fig. 3). It is po.ssible that in types where fine madreporic pores are unknown, some of the larger pores served as madreporic 172 ROBERT TRACY .TAf'KSOX OX ErUIXl. openings. Otherwise all the pores in genital plates doubtless connected with genital glands, as in Recent Echini with accessory pores. In very young Recent Echini, as shown by Loven (1892), there is a single madreporic opening in genital 2 (Plate 2, fig. 3; Plate 3, fig. 14; text-fig. 131, p. 129). Fine madreporic pores soon appear, and in the adult are usually numerous (Strongylocentrotus, text-figs. 132- 139, p. 132). In some types there are few madreporic pores in the adult, a primitive character (Salenia, Plate 4, fig. 1). Usually the madreporite is perforated by many pores, as seen from the interior as well as from the exterior. In the Cidaridae, however (text-figs. 59, 60; 70, 71, pp. 95, 98), while there are many on the exterior of the plate, there is only a single large madreporic pore on the interior, so that at this part of the plate the youthful character is retained in the adult. The same condition exists in Salenia -patter soni (Plate 4, figs. 1,2). In Echinocyamus (Loven, 1874) there is a single external madreporic pore. In Hahrocidaris scutata (text-fig. 206), as shown by Messrs. A. Agassiz and Clark (1908), no fine madreporic pores exist in genital 2, but, instead, there is a single pore in the middle of the plate, which appears to be a retention in the adult of the single madreporic opening characteristic of the young. This is the only case known in an adult regular echinoid. In Palaeozoic Echini a typical madreporite with numerous fine pores is known in a few species. It exists in Lovenechinus lacazei, as observed by Bather (text-fig. 243) ; also, it has been found in Lepidesthes formosa (Plate 68, fig. 5), L. colletti (Plate 71, fig. 1), Meekechinus elegans (Plate 76, fig. 6), and Echinocystites pomum (Plate 18, fig. 6). In a number of Palaeo- zoic genera the apical disc is not known well enough to enable one to state anything with assur- ance. In the Palaeechinidae the apical disc is known in many species and yet the madreporite has been seen in Lovenechinus only, as above noted, and reported in Melonechinus multiporus by Keyes (1894). I have not seen a specimen of Melonechinus with madreporic pores. In post-Palaeozoic regular Echini the madreporic pores exist in genital 2, and are typically limited to that plate. In examining large series of specimens, it is frequentlj' found that the madreporic pores extend beyond genital 2. When this occurs, in the great majority of such variants, they extend to genital 3 and frequently ocular III as well (Centrechinus, text-fig. 188) ; or they may extend to ocular III without invading genital 3 (text-fig. 146, p. 134). This extension of madreporic pores has been seen in many species, but in Strongylocentrotus dro- bachiensis, on account of the number examined, a greater range is known than in others studied. Of this species in 33,000 specimens, 928 cases were noted in which madreporic pores extended from genital 2 to 3 (Plate 5, fig. 13). In 128 cases madreporic pores occur in genital 2 and ocular III (text-fig. 146, p. 134), and in 64 cases in genitals 2, 3 and ocular III, as in Centre- chinus (text-fig. 188). In 80 cases, madreporic pores are in genitals 1, 2, 3, and in 60 cases in genitals 2 and 1 (text-fig. 142, p. 134). Other variations in the extension of madreporic pores beyond genital 2 are only rarely found. In 15 cases, they exist in genital 2 and ocular II; in THE PERIPROCT. • 173 six cases in 2, III, II; in three cases in 2, 3, III, and II; in four cases in 2, 3, II (text-fig. 148, p. 134). In seven cases madreporic pores occur in genitals 1, 2, 3, and ocular III; in four in 1, 2, 3, and II; and in eleven in 1, 2, 3, and II, III (Plate 5, fig. 14). The most extreme range observed was one specimen in which the madreporic pores extended from genital 2 to genitals 1, 3, 4, and oculars III, IV, V. A similar range of variation in the distributipn of madreporic pores and in about the same proportions has been found in other species, especially in the families Echinidae and Strongylocentrotidae. In the young, madreporic pores very rarely extend beyond genital 2, whereas they frequently do in the adult. This change evidently is taken on as the animal increases in size to the adult condition. Occasionally madreporic pores invade interambulacrum 2, and in one somewhat distorted Strongylocentrotus drobachiensis this invasion was extensive. All this goes to show that while typically madreporic pores are limited to genital 2, they may extend from this plate to adjacent genitals, oculars, or interambulacrum, but apparently never to plates of the periproct. They seem never to be absent from genital 2 in Recent regular Echini. When madreporic pores extend beyond genital 2, in the great proportion of cases they move to the left of the antero- posterior axis, emphasizing by this a bilateral axis or plane of symmetry through III, 5. As seen from these studies, the genital plates have nothing to do with the interambulacrum, which develops on either side of the oculars (p. 62). The genitals typically possess genital pores, and one of them possesses madreporic pores, but both of these structures may pierce other parts of the test. Genital plates may, therefore, be considered as structures of secondary importance, of much less morphological value than are the oculars. The Periproct. The periproct is an area of considerable interest. Unfortunately in fossils it is rarely preserved, as the plates easily drop out and are frequently wanting, even in museum specimens of Recent Echini. It is worth noting that in Recent Echini periprocts and peristomes may be rendered strong and safely fastened in place by dipping the specimens in dilute shellac dissolved in alcohol, or gelatine dissolved in water. The same treatment is very eiTective in strength- ening the thin tests of delicate Echini ami in keeping the spines firmly in place. In Palaeozoic Echini the plates of the periproct are small, angular, and completely fill the area, as in cidarids. In Bothriocidaris archaica there are nine small plates that fill the area dorsal to the genitals (Plate 1, fig. 2). In Hyatlechinus beecheri (Plate 25, fig. 5) a few peri- proctal plates are preserved (others restored in the figure), indicating a periproct essentially as in Cidaris. In Palaeechinus (Plate 31, fig. 4), Maccoya (Plate 34, fig. 6), Lovenechinus (Plate 42, fig. 6), and Melonechinus (Plate 56, fig. 1) the periprocts as far as preserved are made up of many polygonal, rather thick plates filling the area, that are essentially like those of 1 74 ROBERT TRACY JACKSON ON ECHINI. Eucidaris tribuloides in structure (text-figs. G2-69, p. 98). In the Lepidesthidae, the peri- proctal plates in the known types are essentially the same, like cidarids, but are rather thinner and more scale-like; such are shown in Lepidesthes formosa (Plate 68, fig. 5), L. coUetti (Plate 71, fig. 1), and Meekechinus elegans (Plate 76, fig. 6). Palaeozoic types as far as known do not throw any light on the suranal plate, which is such a prominent feature in the young of some Recent Echini and the adults of certain genera. There is no evidence that a suranal plate was differentiated in the Palaeozoic, and, as it is apparently wanting in the Cidaroida and Aulodonta, both young and adult, it is also doubtless wanting in the Palaeozoic. In the Cidaroida, periproctal plates are thick and angular, filling the area, with the anus central. They are essentially like those known in the Palaeozoic and in so far may be con- sidered primitive. These plates are shown in representative cidarids in text-figs. 59-74, 164 (pp. 94-99, 149). Loven (1892) figures a young Goniocidaris with a single plate filling the periproct (Plate 2, fig. 3); but Mortensen (1911) thinks this a mistake, as in a specimen of a similar stage he found three plates. Mr. Agassiz's (1904) figures of young cidarids, and those young specimens which I have studied, show no indication of a dominance of a first, over later added plates. It seems, then, that we have here no suranal such as exists in the Saleniidae and in young Echinidae and Strongylocentrotidae, in which groups alone has it been demon- strated. The existence of a suranal plate is apparently a secondary and specialized rather than a primitive character. Dr. Mortensen wrote me in effect that he concurs in this view, and he expresses the same in a recently published paper (1911). In the Centrechinoida the character of periproctal plates presents wide differences in various genera. Of the Aulodonta there are large plates on the periphery of the periproct with small isolated plates within, as seen in Chaetodiadema, Centrostephanus, Astropyga (text-figs, 96-99, pp. 108, 109), or young Centrechinus (text-fig. 88); or there may be small, isolated plates only, with tissue largely leathery, as in adult Centrechinus (text-figs. 93-95, p. 107). Of the Stirodonta in Salenia and Salenocidaris (Plate 4, figs. 1, 6; text-fig. 102, p. Ill) we find a prominent suranal plate lying dorsal to genital 3 and with small anal plates in addition. In the fossil Peltastes (Plate 4, fig. 7) the suranal occupies a prominent place but lies dorsal to ocular III, thus differing from Salenia and young Strongylocentrotus and Echinus, in which the suranal lies dorsal to genital 3. In a series of 74 specimens of Peltastes wrighti one speci- men has two large plates, as in Acrosalenia hemicidaroides (text-fig. 104, p. Ill), and as in that figure the larger plate, which may be considered the suranal, lies against genital 3, as in Salenia. This is a striking difference from the other specimens, in which there is only a single plate lying dorsal to ocular III, as shown in Plate 4, fig. 7, which is the generic character. The fact that the suranal may occupy one of two positions has a certain bearing on other genera. In Acrosalenia spinosa (text-fig. 103, p. Ill) the suranal lies dorsal to ocular III, as in Peltastes, but in A. hemicidiiroides (text-fig. 104, p. Ill) there are two large plates; the larger THE PERIPROCT. 175 of these two, which appears to be the suranal, hes dorsal to genital 3, as in Salenia. In Acro- salenia wiltoni (text-fig. 105, p. Ill) there are four large periproctal plates besides a vacant space, doubtless for small anal plates as in Salenia. The largest of these four is probably the suranal and lies dorsal to ocular III, as in Acrosalcnin spinosa and Peltastes. In Arbacia there are typically four plates in the periproct, l)ut as variants there may be fewer or more than this number. Bell (1879) notes that there may be as few as three or as many as ten, and Mr. Agassiz (1881, p. 57) notes that there may be more than four up to thir- teen periproctal plates. I noted (1899, p. 131) that in Arbacia punctulata there may be as few as three or as many as nine plates as variants from the typical four. In Arbacia the typical four periproctal plates lie in such planes that two are in the antero-posterior axis III, 5, and two are on each side of this axis (text-figs. 111-114, p. 115; text-figs. 107, p. 149; 202). The Y 204 205 205a 206 Text-pigs. 200-20G. — Variation of periproctal plates in Arbacia with the tj'pical character in Habrocidaris. 200. Arbacia punclulala (Lamarck). Woods Hole, Massachusetts. Diam. 25 mm. R. T. J. Coll., 727 Only two periproctal plates; suranal dorsal to genital 3. 201. The same. Diam. 45 mm. R. T. J. Coll., 728. Diam. 47 mm. R. T. J. Coll., 729. Diam. 31 mm. R. T. J. Coll., 730. Diam. 36 mm. R. T. J. Coll., 731. Diam. 43 mm. R. T. J. Coll., 732. X3. 202. The same. X 3. Only three periproctal plates. X 3. Four periproctal plates, the typical character (p. 1 1."").) X 3. Five periproctal plates; compare text-fig. 206. X 3. Six periproctal plates. X 3. Many periproctal plates. Florida. Diam. 32 mm. R. T. J. Coll., 890. X 4. One large and about fifty small periproctal plates. The specimen is so distorted its ocular arrangement is omitted in tabulation of the species, pp. 115, 153. 206. Habrocidaris scutata (A. Agassiz). Santa Cruz, West Indies. Diam. 17.5 mm. Adapted from A. Agassiz and Clark, 1908, Plate 54, fig. 5. X 3. Five periproctal plates; compare text-fig. 203. There is only a single madreporic pore in genital 2 (compare Plate 3, fig. 14). The genital pores lie very far tlorsally (p. 172). 203. 204. 205. 205a. The same. The same. The same. The same. 170 ROBERT TRACY JACKSON ON ECHINI. plate lying dorsal to ocular III is apparently the equivalent of the suranal, as in Acrosalenia wiltoni (text-fig. 105), which also has four large plates. In examining some 2,300 specimens of Arbacia punctulata, considerable variation was found -in the periproctal plates, and they varied in adults from two to many (text-figs. 200-205). Two specimens were found with only two plates and many with three. In both of these forms what I take to be the suranal overlaid genital 3, as in Salenia, not ocular III as usual. A number of specimens were found with five plates and several with more than five, one being shown with six and one with thirteen (text- figs. 204, 205) . In one abnormal specimen from Florida (text-fig. 205a) there is one large peri- proctal plate facing ocular III, as usual, and, in addition, some fifty small rounded periproctal plates of a character similar to those usual in the Echinidae. When there are five periproctal plates in Arbacia, as in text-fig. 203, the condition is exactly like that of Habrocidaris (text- fig. 206). In Parasalenia gratiosa A. Agassiz, as shown by Mr. Agassiz (1873, p. 435), there are four periproctal plates, as in Arbacia, but they lie in a different plane so that two are on each side of the antero-posterior axis through III, 5. Of five specimens of Parasalenia gratiosa seen in de Loriol's collection, in the Geneva Museum, four have the typical four plates in the periproct, but one has only three. Variations of periproctal plates in Arbacia dufresni were also noted as described, p. 115. In the young of the Echinidae and Strongylocentrotidae a suranal plate fills the periproct, as shown by Mr. Agassiz (1874, p. 732). This is typically developed in young Echinus (Plate 3, fig. 14) and Strongylocentrotus (text-figs. 131-134, p. 129). As the animal grows, the suranal gradually loses its prominence, new plates forming along its right posterior border and the suranal retaining while recognizable a position dorsal to genital 3. In the adult of Strongylo- centrolus drobachiensis a plate is often found dorsal to genital 3 somewhat larger than other periproctal plates (text-figs. 135-139, p. 132). It seems that this may be recognized as the suranal still holding a slight supremacy in size. A similar plate similarly located is frequently recognizable in other genera, as Toxopneustes (text-fig. 122, p. 122) and Echinus (text-fig. 115, p. 117), and may fairly be considered as the suranal. There is considerable individual variation, some specimens having no such larger plate at this area and others possessing one. Dr. Mortensen (1911) expresses the view that the suranal is not a primitive structure form- ing an essential part of echinoid morphology, but is a specialized structure found only in certain groups of Echini. I heartily agree with this view. It seems that the suranal is simply one of the plates of the periproct which has attained exceptional prominence and which in youth entirely fills this area in those families in which it is a marked feature (Saleni- idae, Echinidae, Strongylocentrotidae). In the adults of the Temnopleuridae, Echinidae, Strongylocentrotidae, and Echinometridae (text-figs. 115-161) the periproctal plates are usually relatively large and solid, more or less numerous, but not definitely limited to a small number (except in Genocidaris and Parasalenia), ARISTOTLE'S LANTERN AND PERIGNATHIC GIRDLE. 177 and fill the periproctal area. In irregular Echini periproctal plates are relatively numerous and always fill the area completely. As clearly shown by Mortensen (1907, Plate 13) in Brisaster (Schizaster) fragilis, the periproct in the very young individuals is within the apical disc, as in regular Echini. It early travels out and soon assumes its adult position in the posterior inter- ambulacrum. In regular Echini the periproct is typically enclosed by a ring of genital, or genital and ocular plates. In the Echinothuriidae, on account of the separation of genitals and oculars, the periproct maj^ come in contact with the interambulacra (text-fig. 170, p. 149). As a rela- tively rare variation, a similar condition may exist in other regular Echini, as Sirongylocentrotus gibbosus (text-fig. 156, p. 145), S. drobachiensis (Plate 5, fig. 15), and S. lividus (Plate 6, fig. 5). In such types the young interambulacral plates develop in contact with the oculars as usual, and the periproct has no relation with the interambulacra excepting that of mechanical contact (pp. G3, 110). In the Exocycloida the periproct lies in interambulacrum 5; it is therefore separated from the ocular and genital plates of the apical disc and completely surrounded by interambulacral plates. The Aristotle's Lantern and Perignathic Girdle. Comparatively little has been pubhshed in regard to the Aristotle's lantern in Palaeozoic Echini. Trautschold (1868) figured complete lanterns in Archaeocidaris rossica, SoUas (1899a) figured a lantern in Palaeodiscus, and Meek and Worthen (1873) in Lepidocidaris in part. The lantern and its muscles in living Echini have been worked out quite fully by a number of authors, the most important being Valentin (1841), A. Agassiz (1872-'74, 1904, 1908, 1909), T. H. Stewart (1861), and Lov4n (1892). The jaws and muscles are often very inadequately or incorrectly given in text-books. For comparison with fossils, a lantern with soft parts was worked out iij Strongylocentrotus (Plate 5) as a basis for comparison. The lantern of this genus and a number of others not previously described were studied, and are given more or less fully in the following pages. The perignathic girdle might properly be described with the lantern in the several types, but it seemed best to treat it separately (pp. 189-198). It is believed that the structure of the lantern is of great value in systematic classification, and that the structure of its several parts presents characters that are of ordinal or subordinal value. As Dr. Mortensen pointed out (1904, p. 54), the structure of the teeth, keeled or un- keeled, is "a very important character, though it has hitherto received very little attention." Besides the teeth there are other features of value. Briefly stated, the essential points are: teeth grooved or keeled ; epiphyses narrow, or wide and united by suture ; the top of the pyramids, as seen when the epiphyses are removed, a smooth floor, or pitted ; foramen magnum deep, or shallow; angle of outline of the lantern depressed or erect; compasses present or absent. 178 ROBERT TRACY JACKSON ON ECHINI. The lantern of Strongylocentrotus drobachiensis is taken up first as a type of structure, because opportunity offered to study living material and abundant fresh specimens for dis- section. The structure has not been published in this type excepting in so far as Lov6n (1892) described the jaws and muscles in the young; the study has, therefore, interest from making known the structure in a common type and adds some features not previously recorded. In Strongylocentrotus there are five keeled teeth lying free about the mouth. The tooth is held in place by the two half-pyramids which ventrally enclose it closely by an inward extension of the base of the lateral wings (Plate 5, fig. 5). The tooth rests against the dental slide (s. d.) on the inner face of the pyramids. On each side the dental slide is produced dorsally as a guid- ing or styloid process, which in face view is seen projecting above the base of the foramen mag- num (Plate 5, figs. 4, 7). After traversing freely the space of the foramen magnum, in this type the tooth rests against two guiding processes, the crests of the epiphyses (Plate 5, figs. 2-5, 7, 9). The tooth then curves inward, takes a second sharp curve on itself and in the same plane, and terminates in a free point in the dental capsule (Plate 5, fig. G). From the mouth to a point just above the crests the tooth is calcified; beyond this point to the proximal tip the tooth is soft and pulpy and easily destroyed. In area 1 of Plate 5, fig. 9, the soft part of the tooth has been removed, whereas it is in place in areas 2 and 3. The calcified part is that usually shown in figures of lanterns, and is the only portion preserved in fossils, as in Archaeocidaris (Plate 12, figs. 4-8) and Pholidechinus (Plate 27, figs. 4-6). An entire tooth spread out fiat is shown in Plate 5, fig. 8; it is seen that at the dorsal, young or growing portion of the tooth it is grooved, but, passing ventrally, the character of the keel is soon taken on. That is, the young, last added portion of the tooth as a localized stage in development is grooved, as is the whole tooth in the Aulodonta, Cidaroida, and Palaeozoic forms; whereas the older or earlier formed portion of the tooth has taken on the full specific and subordinal character of a keel. The pyramids are each composed of two pieces joined by a median or pyramidal suture (Plate 5, figs. 4, 7, p. s.), and each piece is spoken of as a half-pyramid. Dorsally the pyramid in- cludes a wide angle, the foramen magnum, which in this and allied species is rather deep. On the inner side at the base (Plate 5, fig. 5) the wings of the pyramid curve around so as to embrace the tooth tightly at its point of exit. Each half-pyramid consists of a peripheral portion, the outer face of which gives rise to two muscles, and the inner bears one of the two dental slides which support each tooth, and a lateral wing which is strongly ridged horizontally for attach- ment of the interpyramidal muscles. The dorsal face of the half-pyramid, seen only when the epiphysis is removed (Plate 5, figs. 9, 10), presents a series of pits of considerable size and depth. These pits have apparently been overlooked by previous observers. They are important, as they are characteristic of the order of the Centrechinoida, but are not found in other orders of Echini. In 100 specimens of Strongylocentrotus drobachiensis from Frenchman's Bay, Maine, examined for this character, pits exist in all the pyramids, but there is considerable variation as regards the pattern and depth of the pits; occasionally they are very shallow. ARISTOTLE'S LANTERN AND PERIGNATHIC GIRDLE. 179 An epiphysis surmounts each half-pyramid to which it is joined by close suture (Plate 5, figs. 2-7, 9). The epiphysis extends proximally, capping the half-pyramid, and laterally ex- tends over the foramen magnum and meets its fellow of the opposite side in a median suture. Each epiphysis presents a glenoid cavity and an internal and an external tubercle (Plate 5, figs. 3, 5, 9), which serve in articulation with the brace. It also bears an elevated process or crest which is apposed to the tooth and dorsally supports that organ. The extension of the epiphyses over the foramen magnum so as to unite in suture and the crests developed on the same, ai-e important characters seen in tlie four families of the Temnopleuridae, Echinidae, Strongylocentrotidae, and Echinometridae, comprising the new suborder Camarodonta, but are absent in other Echini (p. 183). The brace is a block-shaped plate, often called rotula, which rests on and interlocks with the two opposed epiphyses (Plate 5, figs. 9, 11, 12). On its outer lateral borders it presents two condyles which fit into the glenoid cavities of the epiphyses. The compass (Plate 5, figs. 2, 9) rests on top of the brace, extends over its whole length, is attached to the brace on its inner proximal end by a tiny ligament, and is bifurcated on its outer end. Each compass consists of two parts, a suture just within the circular compass muscles separating the compass into an inner and an outer piece. The structure of the compass and brace is quite uniform in Echini, excepting the clypeastroids. There are thus forty pieces in the lantern of Strongylo- centrotus, and in all other Echini that possess a lantern, excepting the clypeastroids. These parts are in brief : five teeth ; ten half-pyramids ; ten epiphyses ; five braces ; ten pieces making up the five compasses. Next taking up the soft parts associated with the lantern of Strongylocentrotus drdbachiensis, we find that each tooth at its basal poi'tion is enclosed in a voluminous, very delicate trans- parent sac that may be called the dental capsule. A dental capsule is figured by the Sarasins (1888) in Asthenosoma,' and is mentioned in Cidaris by T. H. Stewart (1861) and in Echinus by Chadwick (1900), but I think it has not been adequately shown before. In order to see the sac properly, a specimen should be opened alive, right out of the sea. It is so delicate and sensi- tive, that if opened a few hours later, though the animal be still active, the sac has shrunk to small proportions. In alcoholic material as far as observed it is always collapsed. When opened thus alive, the capsules are so inflated that they look like five bladders radially arranged, and so large that they actually touch one another at the area nearest the oesophagus (Plate 5, fig. 1). The sac envelops the base of the tooth completely, lies free, but on its outer border passes over the epiphyses and some distance down the face of the area of the foramen magnum (Plate 5, fig. 6). Similar capsules were seen in fresh specimens of Arbacia ipunctulata, Sphaer- echinus granularis, and Strongylocentrotus lividus, and very small ones in Echinarachnius parma. It would be interesting to study the capsules in perfectly fresh material of other Echini. ' Unfortunately in Lang's (1896) copy of this figure, tlie dental capsule is labeled Polian vesicle. 180 ROBERT TRACY JACKSON ON ECHINI. The muscles of the lantern are very complex. They have not been described in adult Strongylocentrotus, though Loven (1892) showed them in a very young one and in exquisite detail in the adult of several types. There are sixty muscles in Strongylocentrotus and other regular Echini. There are ten protractors (Plate 5, figs. 1, 4, pr.; text-fig. 229), which take origin on the upper outer face of the half-pyramids and epiphyses and are inserted at the base of the two half-interambulacra which are associated with the intervening ambulacrum (compare text-figs. 218, 219, p. 191). The protractors serve to extend the mouthparts, which the animal actively does when in good condition. The ten retractors (Plate 5, figs. 1, 4, re.), which serve to open the jaws, take origin on the lower outer face of the half-pyramids and are inserted on the auricles. The retractors of the two half-pyramids that are united by the interpyramidal muscle pass to the two auricles of the same ambulacrum (compare text-fig. 219, p. 191). Ten radial compass muscles take origin from the bifurcated termini of the compasses and are in- serted at the base of the two next adjacent half-interambulacral areas. From studying their motions in life, and from their position and wide divergence, it seems that the function of the radial compass muscles is to maintain and restore the vertical position of the lantern. When alive, the mouth with the peristome is freely and actively extended and retracted, and not in a vertical plane only, but with much sidewise or inclined motion, so that the radial compass muscles are in just the position to effect the motion and restore the perpendicular. It is note- worthy that in clypeastroids, where the lantern is in almost immediate contact with the base of the test and little motion is possible, the protractor muscles are very small and the compasses with their muscles are absent. The distribution of protractor, retractor, and radial compass muscles is immediately con- nected with the view expressed (pp. G2, 190), that the corona may be considered as made up of five rather than ten areas. Five large but short interpyramidal muscles (Plate 5, fig. 2, ip.), extend from the corrugated lateral wing of each half-pyramid to the similar wing of the next adjacent half-pyramid. These muscles serve to close the jaws and from their size must be very powerful. Ten tiny internal brace muscles (Plate 5, fig. 12, i. b.) extend from the brace to the two associated epiphyses, and ten similar external brace muscles, (e. b.), extend also from the brace to the epiphj^ses, binding these parts together and doubtless giving some mobility. Five circular compass muscles (Plate 5, fig. 1, i.) extend between the compasses, being inserted at the proximal end of the outer of the two pieces of which each compass is composed. Their function is not evident. In brief the sixty lantern muscles of Strongylocentrotus and other regular Echini are as follows: ten protractors; ten retractors; five interpyramidal muscles; ten internal brace muscles; ten external brace muscles; five circular compass muscles; ten radial compass muscles. Such being the character of the lantern of the adult, it is important to see what is the ARISTOTLE'S LANTERN AND PERIGNATHIC GIRDLE. 181 character in the young. Loven (1892, p. 13, Plate 4) described the lantern of a young Strongy- locentrotus drobachiensis only 1.2 mm. in diameter. At this stage the pyramids are wide-angled, so that the whole lantern is broadly inclined instead of being nearly perpendicular, as in the adult; also the sides of the half-pyramids bearing the interpyramidal muscles are curved instead of being in a nearly vertical plane as in the adult, and as a result the interpyramidal muscles are relatively longer than in the adult. In these several characters the lantern of young Strongylocentrotus point for point agrees with the typical Palaeozoic lantern as seen in Archaeo- cidaris (Plate 12, figs. 4-6) and Pholidechinus (Plate 27, figs. 4-6). The same agreement with the Palaeozoic characters is seen in the lantern of young Goniocidaris (Plate 2, fig. 17). Having considered the lantern and muscles of Strongylocentrotus, representative lanterns of Echini will be taken up in the sequence of their structural differentiation. Of Bothriocidaris nothing is known but the tips of the teeth, or pyramids. They lie opposite the ambulacra (Plate 1, fig. 1). That they occupy this anomalous position is without explanation unless by an accidental twist they have come to assume it. It is the most ambiguous feature of this important type. I have been so fortunate as to secure for study some excellent lanterns of Palaeozoic genera. These are described under their several species, but are briefly considered here. The essential character of the lantern in the Echinocystoida and Perischoechinoida is represented by Archaeocidaris (Plate 12) and Pholidechinus (Plate 27) ; other genera as far as known present only slight differences. Of other genera it is known more or less completely in Palaeodiscus, Oligoporus (Plate 50, figs. 11, 12), Melonechinus (Plate 56, figs. 9, 10), Lepidesthes (Plate 68, figs. 9-14), Pholidocidaris (Plate 74, figs. 2, 6, 7), and Meekechinus (Plate 76, fig. 7). In the Palaeozoic genera the teeth are grooved, and when in place, extend slightly above the base of the foramen magnum (Pholidechinus, text-fig. 207, Plate 27, figs. 5, 6). While the teeth are usually obtusely acuminate orally, as in Pholidechinus and Lepidesthes (Plate 68, fig. 9) , in one genus, Meekechinus (Plate 76, fig. 7), the teeth are deeply serrate distallj^ a unique character. The pyramids are wide-angled in outline and as a result the lantern is inclined at an angle of about 45 degrees instead of being nearly perpendicular as in the adults of most living regular Echini; also the lateral wings of the pyramids are curved, indicating quite long interpyra- midal muscles (Plate 27, figs. 4, 6). In these characters the Palaeozoic lantern approaches closely that in the young of modern types (Goniocidaris, Plate 2, fig. 17), and also the adult of Recent echinothuriids (Plate 2, figs. 19-21). The foramen magnum is moderately deep (Archaeocidaris, text-fig. 208, Plate 12, fig. 4). It appears very shallow in Pholidechinus (Plate 27, fig. 6), but this is in part due to the position in which the specimen was drawn. The lateral wings of the pyramids present a corrugated surface for attachment of interpyra- midal muscles (Plate 12, fig. 8; Plate 13, fig. 13; Plate 68, fig. 12). The dorsal surface of the half-pyramids, seen when the epiphyses are removed, is smooth, as in the Cidaroida 182 ROBERT TRACY JACKSON ON ECHINI. (Plate 2, fig. 9), not pitted as in tiic Centrechinoida. The half-p3'ramid seen from within shows a dental slide for the support of the tooth (Plate 13, fig. 14; Plate 50, fig. 12), as in Recent Echini. The styloid processes do not extend above the base of the foramen magnum, in this feature being like the Cidaroida, but unlike the Centrechinoida, where the styloid processes are visible in face view (text-fig. 211). The epiphyses cap the half-pyra- mids and are narrow, projecting only slightly beyond the limits of the outline of the half- pyramids (text-figs. 207, 208; Plate 12, figs. 1, 7; Plate 27, figs. 4-6). Each epiphysis pos- sesses a glenoid cavity for the interlocking of the brace (P