OCULAR AND GENITAL PLATES. 



109 



fig. 97), Astropyga (text-fig. 99), and Chaetodiadema (text-fig. 98). There are 33 aberrants, 

 3 %. Thirty-one of these have I, V, IV, III insert. This character is common in cidarids as 

 discussed, but is rare in the Centrechinoida, most of the cases seen occurring in this species. 

 One aberrant has oculars I, IV insert, V being excluded by the fusion of genitals 4, 5, as in text- 

 fig. 144; and one aberrant has V, IV, III insert, evidently an imperfect I, V, IV, III, and a 

 very rare variant, which, however, is the species character of Strongylocentrotus gibbosus (text- 

 fig. 15G). While three plates insert is strongly the character of C. selosiis, in no case were oculars 

 I, V, II insert, which is so markedly a character of the Echinidae and Strongylocentrotidae. 

 It is a remarkable fact that the aberrants of this species fall so definitely in one place, I, V, IV, 

 III insert, there being only three exceptions to this in the whole 1,278 specimens examined. 



In Echinothrix calamaris (11 specimens) oculars are all insert in ten, and in one I, V, IV are 

 insert like the typical character of Cenlrechinus setosus. In Echinothrix diadema (18 specimens) 

 all oculars are insert. 



Centrostephanus asteriscus is interesting as the only Recent species of the Centrechinidae 

 in which the oculars are all exsert in the adult, as they are typicallj- in Jurassic species of the 



Text-figs. 99-101. — Ocular plate arrangomenf, in Confrodiinidae and Echinothuriidao. 



99. Astropyga pulvinala (Lnmarck). Gulf of California. Diam. 94 mm. R. T. J. Coll., 740. X 4. Oculars all 

 insert. 



100. The same, section of ambulacral plates shewing .adoral imbrication, and of interambulacral plates showing aboral 

 imbrication (p. 74). 



101. Aaihcnnsoma ijimai Mortensen. Tokyo, Japan, 55 fath. Diam. 105 mm. II. T. J. Coll., 702. Oculars all in- 

 sert, genitals, split up, especially ventrally (p. 168). 



