DIADEMAD^. 83 



herewith subjoin ; the genera referred to this family are divided into four groups, 

 based upon the structure of the tubercles, whether they are perforated or not perforated, 

 and crenulated or not crenulated. ■ 



In the CiDARiDiB these characters have not much significance, and are present or 

 absent in many species of congeneric forms ; in the DiADEMADiE, however, they arc more 

 stable and persistent, and have served to form a great number of genera. If from an 

 organic point of view this structure of the tubercles is only of secondary importance, in a 

 palseontological sense it affords a character which is readily seen, and nearly always 

 well preserved. 



The first group comprehends the Diademad^e^ with tubercles perforated and crenu- 

 lated : Hemicidaris, Agassiz ; Acrocidaris, Agassiz ; Pseudodiadema, Desor ; JDiadema, 

 Gray ; Hibertia, Michelin ; Microdladema, Cotteau ; Heterodiadenia, Cotteau ; Astero- 

 cidaris, Cotteau ; Gli/phoci/phus, Haime. 



The second group includes the genera with tubercles perforated and not crenulated : 

 Cidarojjsis, Cotteau ; Diademopsis, Desor ; Hemipcdina, Wright ; Echinopsis, Agassiz ; 

 Orthopsis, Cotteau : Pedinojms, Cotteau. 



The third group is destined to receive the genera which have the tubercles imperforated 

 and crenulated : Cypliosoma, Agassiz ; Micropsis, Cotteau ; Tcmnojjieuriis, Agassiz ; Echi- 

 nocyjjhis, Cotteau. 



The fourth and last group contains the genera with tubercles imperforated and uncre- 

 nulated: Goniojjygus,h.%^'&'&YL; Acropeiiis, Agassiz; Zeiosoma, Cotteaii; Uc/nnocidaris,Des- 

 moulins ; Codopleurus, Agassiz ; Kceraiaphorus, Michelin ; Codiopsis, Agassiz ; Cottaldia, 

 Desor ; Magnosia, Michelin ; Glypticus, Agassiz ; Temnechinus, Forbes ; Opechinus, Desor. 



The genera which compose these four groups arc distinguished by straight or fiexuous 

 ambulacra, the disposition of the tubercles, the structure of the apical disc, the sutural 

 and angular impressions which mark the ambulacra! and inter-ambulacral plates, the 

 comparative width of the peristome, and the form and structure of the spines. 



The following table contains a definition of the opposable characters of the thirty -one 

 genera composing the family Diademad^. 



^ In my classification of the Echinid/E I have separated Hemicidaris and Acrocidaris as a distinct 

 family, the HEMiciDAUiDiE. 



