OCULAR AND GENITAL PLATES. 



95 



In the tables of ocular plate arrangement, (pp. 100, 101, 142, 143, 154-163) are shown numeri- 

 cally the typical character of the species listed with the normal arrested and progressive vari- 

 ants, and in the right-hand column the number of aberrant variants, the details of which for 

 the Centrechinoida are separately listed in the final table (p. 164). In the numbers given 

 in the tables the denominator, in Arabic numerals, shows the number of specimens in which 

 the given character was found anil the numerator, in italic numerals, expresses the per- 

 centage which this number bears to the total number of specimens examined. In the 

 last table are shown the characters of all aberrant variants tabulated in the Centrechinoida. 

 It is noteworthy that in only a few species are such aberrants common, and it is a curious fact 

 that has no obvious explanation, that most of such species occur off the west coast of South 

 America. They are, namely, Arbacia spatuligera and nigra, and Strongylocentrotus albus and 

 gibbosus. In the table of aberrants (p. 164) it is a striking fact as there brought out that certain 

 aberrant variations are a characteristic of definite species or families in the order. In all of 

 the tables of ocular plate arrangement the species within the genus are arranged on the basis of 

 the number of oculars insert or its frequency as a variant. When no difference was observed 

 they are arranged alphabetically. In the text they are considered in the same sequence. 



The Cidaroida of the Mesozoic rarely have the oculars and genitals preserved, but they 

 were found in seven specimens, all alike in character. The oculars are all exsert in a choice 

 specimen of Cidaris florigemma from the Coral Rag, near Calne, in the Jermyn St. Museum; 

 and in two specimens of Cidaris sceptifera, from the Chalk, one being in the Jermyn St. and the 

 other in the British Museum. The same structure is shown in Cidaris coronata (text-fig. 



n IV 



58 



Text-figs. 58-tiO. — Ocular plaLe iinangeiuent in Cidaridae. 



58. Diplocidaris desori Quenstedt. White Jura, Sonntheini. 



59. Cidaris affinis Philippi. Naples Station. Diam. 30 mm. 

 character of this species. 



60. The same specimen, internal view. All oculars exsert as in the young and in the Jurassic. The madreporite 

 has one large madreporic pore as in the young (Plate 2, fig. 3), not many pores as on the exterior of same plate. 



Stuttgart Museum Coll., 9,778. All oculars exsert. 

 R. T. J. Coll., 643. X 3. All oculars insert, the local 



