86 PSEUDODIADEMA 



D. Tubercles not perforated and not crenulated — continued. 



yyy. Inter-ambulacral tubercles irregularly arranged above the ambitus, often 



lacerated ...... Glypticus. 



h. Ambulacral and inter-ambulacral plates provided with angular and sutural 



impressions. 



X. Impressions angular ..... Tjsmnechinus. 



XX. Impressions sutural, and angular, and much more defined . . Opechinus. 



The stratigraphical distribution of the Diademadse extends from the Trias to the 

 modern epoch, where a few species now live in tropical seas. Of the thirty-one genera 

 enumerated in the above table, seven are proper to the Oolitic period : Microdiadema, 

 Asterocidaris, Cidaropsis, Heinijjedina, Acropeltis, Glypticus. Seven to the Cretaceous 

 period : Heicrodiadema, GJypliocyphus, Orthojjsis, Pedlnopisis, Echinocyphus., Leisoma, 

 Codiopsis. Five are special to the Tertiary period : Hibertia, EcJiinopsis, Ccelopleurus, 

 TemnecJiinus, Opechinus. Three to the Modern period : Diadema, EcJdnocidaris, and 

 Karaiaphorus. One genus, Pseudodiadema, is common to the Oolitic, Cretaceous, and 

 Tertiary periods. Three genera are found in the Oolitic and Cretaceous periods ; Henii- 

 cidaris, which commenced in the Trias, Acrocidaris and Maynosia, but neither extend above 

 the Neocomian. Pour genera are common to the Cretaceous and Tertiary periods : 

 Goniopyyus, Cotfcddia, Cyphosoma, and Micropisis. Tlie genus Temnopleurus appeared 

 in the Tertiary period and exists in our present seas. 



Pseudodiadema, Desor. 1854. 



This genus is composed of small Urchins with a moderately thick test, which rarely 

 attains two inches in diameter ; the ambulacral areas in general are one third or even one 

 half the^ width of the inter-ambulacral areas; the primary tubercles of both areas are 

 perforated, and nearly all of the same size ; the bosses are small, and have sharply crenu- 

 lated summits. 



The ambulacral areas have two rows of tubercles ; the inter-ambulacral areas two 

 rows only, or two rows of primary and two or four short rows of smaller secondary 

 tubercles, or they have four, or six rows of nearly equal-sized primary tubercles at the 

 ambitus. 



The poriferous zones in general are narrow and straight ; the pores in one section are 

 unigeminal throughout, and in another they are bigeminal in the upper part of the zones. 

 The apical disc is small ; and the anterior ovarial plates are larger than the posterior pair. 



The mouth-opening is large, the peristome deeply notched, and the oral lobes are 

 nearly equal. 



