86 PSEUDODIADEMA 



D. Tubercles not perforated and not crenalated — continued. 



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



lacerated ...... Glypticcs. 



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

 impressions. 



,x. Impressions angular ..... Temkecuixl'S. 



XX. Impressions sutural, and angular, and much more defined . . Opecuis'US. 



The stratigraphical (li.-;tribution of the Diadeniadse 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, 

 Aakrocidarlx, Cidaropsis, Heiuipedlna, AcropeUin, GlypticMS. Seven to the Cretaceoas 

 period : Ileterodiadema, GlyphocyphuH, Orlhopsiis, Pedinopsix, Echinoci/phm, Leisoma, 

 Codiojjsis. Five are special to the Tertiary period : Jlibertia, Echinojms, Cceloplevrm, 

 TemnecJdnm, OpecJdnus. Three to the jNIodern period : Diadema, Echinocidaris, and 

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

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

 c2V«r/y, which commenced in the Trias, ^croczV/ar/s and J/«(7«o*?ff, but neither extend above 

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

 Goniopyyus, Collaldiu, Cyphosoma, and Micropms. The genus Temnopleurm 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 

 attiiins two inches in diameter ; the ambulacral areas in general arc 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. 



Tlie ])oriferous zones in general are narrow and straight ; the pores in one section are 

 unigcniinal 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 motith-opening is large, the [)eristoine deeply notched, and the oral lobes are 

 nearly equal. 



