OrULAR AND GENITAL PLATES. 



103 



relatively rare in the Cidaroida. The character of the apical disc in the Palaeozoic Cidaridae 

 (Miocidaris) is unknown and would be most interesting to ascertain. 



Turning to the Centrechinoida, it is found that in this order the arrangement of the ocular 

 plates as regards insertness is much more definite and constant than in most of the Cidaroida. 

 Taking up the first suborder Aulodonta, we find that in the Mesozoic Hemicidaridae the lead- 

 ing character is for all the oculars to be exsert, as shown in the first two species listed in the 

 table. In Hemicidaris intermedia (169 specimens) the oculars are exsert in 89% (text-fig. 75), 

 but 13 specimens, 8%, are progressive variants with ocular I insert, and in five specimens, 3%, 

 the bivium is insert. In H. crenularis (58 specimens) 81 % have all oculars exsert, but 16 % 

 are progressive variants with ocular I insert, and 2% progressive with oculars I, V insert, like 

 the typical character of the next higher species. One specimen is aberrant with ocular III 

 alone insert, a very rare variation seen in only two other sea-urchins (table, p. 164). 



In Hemicidaris luciensis typically, 56%, the bivium is insert (text-fig. 77) like the most 

 progressive variants of H. intermedia. Seven specimens in 16, however, are arrested variants, 

 two with all oculars exsert, the dominant character of the genus, and five with ocular I insert 

 (text-fig. 76), like the progressive variants of Hemicidaris crenularis and intermedia. Hemi- 

 cidaris langrunensis likewise has typically the bivium insert, 67 %, as a species character, 

 but of 18 specimens, 11% have all the oculars exsert and 22% have ocular I only insert, as 

 arrested variants. In Hemidiadema stramonium (10 specimens) the oculars are typically all 

 exsert, 90%, but 10% are progressive with ocular I insert. In Astcrocidaris minor (text-fig. 

 78), Goniopygus peltatus, and Glypticus hieroglyphicus (text-fig. 79) the oculars are strongly 

 exsert in all of the 134 specimens seen (table, p. 154). 



m ^ m 



nr 



m 



Text-figs. 75-79. — Ocular plate arrangement, in the Heniieitlaridae. 



75. Hemicidaris inlcrmcdia {Fleming.). Coral Rag, Calnc, Wilts, England. Diam. 32 mm. R. T. J. Coll., 720. All 

 oculars ex.sert, the typical character. 



76. Hemicidaris luciensis d'Orbigny. Jurassic, Bath sur Mer. Diam. 22 mm. R. T. J. Coll., 71S. Ocular I insert, 

 an arrested variant. 



77. The same. Diam. 25 mm. R. T. J. Coll., 719. Oculars I, V insert, the typical character. 



78. Aslerocidnris minor CoiicsM. Jura.ssio, Sollies, France. G. Boehm Coll., Freiburg i. B. Oculars all exsert. 



79. Glyplicus hieroglyphicus Agassiz. Coral Rag, St. Pierre, Ncuchatel. Diam. 19 mm. R. T. J. Coll., 606. Oculars 

 all exsert. All figures X 4. 



1 The suborders of the Centrechinoida are new divisions defined and discussed in the section on classification and p. 183. 



