104 



ROBERT TRACY JACKSON ON ECHINI. 



HI 



Text-fig. 80. — Dermatodiadema 

 anlillarum A. Agassiz. West In- 

 dies, 955 fathoms. Diam. 9 mm. 

 R. T. J. Coll. 670. X 6. All 

 oculars broadly insert. 



Of the Aspidodiadematidae 123 specimens, including four species, have been examined. 

 In all the oculars are widely insert, as in Dermatodiadema antillarum (text-fig. 80) . The oculars 

 are relatively exceptionally broad and large, as pointed out by Mr. Agassiz (1904, p. 59, 75), 



and, as he notes, are comparable to the Palaeechinidae in that 

 all meet the periproct. They differ, however, in their relatively 

 great size, especially their width, as compared with the genitals. 

 The genitals are peculiar in that the pores are large and nearly 

 or quite in the middle of each plate. The apical disc in the 

 Aspidodiadematidae is relatively very large, being about 30 to 

 60 % of the diameter of the test. In this it differs radically 

 from the Palaeechinidae in which the apical disc is relatively 

 small, usually being less than 20 % of the diameter of the test. 

 In the Mesozoic Centrechinidae the ocular plates are all 

 exsert typically, in strong distinction to living species of the 

 family. Of 18 specimens of the Jurassic Pseudodiadema pscu- 

 dodiadema, in 89 % all oculars are exsert (text-fig. 81), but in 11 % 

 ocular I is insert (text-fig. 82), a progressive variant. It is of in- 

 terest to note that this ocular, I, is the first to become insert 

 in the development of the recent Centrechinus (text-fig. 90). In the Jurassic Magnosia (text- 

 fig. 83), Cottaldia, Phymechinus (text-fig. 84), and Pedina, all oculars are exsert as usual in 

 Mesozoic species of the family excepting one specimen of Phymechinus in which the bivium 

 is insert. Of Jurassic Stomechinus several species, including 82 specimens, were studied. 

 Usually all the oculars are exsert as in S. bigranularis (text-fig. 85). In a few cases, six in all, 

 ocular I or V, or I and V, are insert as in text-fig. 86. These are distinctly progi'essive variants 

 for the species and the geological horizon, and are comparable to the second and third stages 

 in the development of oculars in recent Centrechinus, where one ocular or the bivium is insert 

 as in text-figs. 90, 91 (table, p. 155). 



In the Mesozoic Polycyphus and Orthopsis (text-fig. 87), oculars were all exsert in the 

 51 specimens seen, except that in one specimen of P. normannus ocular I is insert. 



In Centrechinus setosus we have the type species of the family and therefore one of much 

 importance. Mr. Agassiz early recognized several species of the genus; in the Revision he 

 recognized only two. Dr. Mortensen (1904) revived the earlier species and Messrs. A. Agassiz 

 and Clark (1908) followed him in taking up the older divisions of the genus. Having had speci- 

 mens of the several species, I confess I cannot recognize the specific distinctions as diagnostic 



' Text-figs. 78 and 82-84 were drawn from specimens in (he personal collection of Professor G. Boehm of Freiburg i. 

 B., who kindly gave the opportunity to study them and other material. 



^ For the change of name from Diadcma sclosam to Cfulnxhiiius xctosus, see footnote pp. 27, 28. 



