ECHINODERMATA. ECHINOIDEA. 81 



Periproct large, placed between the peristome and the 

 posterior margin of the test. Inferior Oolite to Corallian. 

 Pygaster. Test large, depressed, outline pentagonal 

 or circular, base concave. Apical disc small, madre- 

 poric plate large. Ambulacral areas straight, simple; 

 pores unigeminal ; tubercles in vertical rows. Interam- 

 bulacral areas wide, tubercles perforate, crenulate or not. 

 Peristome central, large, decagonal. Jaws present. Peri- 

 proct very large, placed behind the apical disc. Inferior 

 Oolite to Cretaceous. 



Echinobrissus (= Nucleolites). Test depressed ; out- 

 line oval or quadrilateral, rounded anteriorly, truncated 

 and broadest posteriorly ; inferior surface concave. Apical 

 disc compact, four perforate genital plates, and one 

 imperforate. Ambulacral areas subpetaloid, pores uni- 

 geminal, the outer pore elongated. Interambulacral 

 plates wide, tubercles small. Peristome oval or pentagonal, 

 excentric, a little anterior. Jaws absent. Periproct 

 placed in a sulcus on the dorsal surface. Inferior Oolite 

 to Recent. 



Echinocorys (= Ananchytes). Test very convex 

 above, inferior surface flattened, outline oval. Apical 

 disc a little elongated ; the four genital plates perforated, 

 the two anterior separated from the two posterior by two 

 ocular plates. Ambulacral areas simple, pores unigeminal. 

 Interambulacral plates large, tubercles small. Peristome 

 bilabiate, placed anteriorly. Jaws absent. Periproct 

 oval, inframarginal. Upper Chalk. 



Holaster. Test cordiform, inferior surface more or 



less flattened, superior surface with a broad shallow 



groove anteriorly. Apical disc elongate, the two pairs of 



genital plates separated by the two oculars. Ambulacral 



w. p. 6 



