THE OOLITIC ECHINOUERMATA. 



417 



milling of the ring, — are all details of structure which individually and collectively aftbrd 

 good specific characters, as they are persistent details which are more or less developed on 

 every considerable fragment of the test and spines of Echinoidea. 



Taking these characters for our guidance, I have grouped the genera, already so nume- 

 rous by the discovery of extinct forms, into the following natural families : 



A TABLE, SHOWING THE SECTIONS AND FAMILIES OF THE ECHINOIDEA. 



ORDEK. 



Order— ECHINOIDEA 



SECTIONS. 



Section A. 



I 



Echinoidea endocyclica. 



Vent •within the genital plates, 



always opposite the mouth- 



Section B. 



Echinoidea exocyclica. 



Vent ■without the genital plates, 



never opposite the mouth. 



FAMILIES. 



ClDARIDyE. 



HEillCIUARID.Ii. 



DlADEMAD^. 



echixid-e. 

 Saleniad^. 



EcHINOCONIDiF.. 



COLLYRITID-E. 



EcHINONIDiE. 



Echini nthidjE. 



ECHINOLAMPIDiE. 

 ClYPEASTERIDjE. 



echinocoridj,. 

 Spatangid.e. 



