308 



THE ECHINOIDEA 



Genera Arbacia, Gray ; Echinocidaris, Dune, non Desmoulins ; Coelo- 

 pleurus, Ag. ; Podocidaris, A. Ag. 



SUB-ORDER 3. DIADEMIXA. 



Diademoida including a series of Echinids in which the compound 

 umbulacral plates gradually increase in complexity. In the simplest 

 forms all the plates are simple primaries ; in others, some of the plates 



o 

 o o o 



O O 



Flo. XXI. 



Arbacia nigra. 1, ainbulacral plate ; L', interainbiilaoral plate showing articular 

 pits (ji) and knobs (/,). 



are compound, each being formed of three primaries (the diademoid 

 type) ; in others, again, some of the plates consist of three or more prim- 

 aries and one or more demi-plates, which occur between the aboral and 

 middle primaries. 



This is the largest of the sub -orders of regular Echinoidea and 

 includes important families. It represents an evolutionary series from 

 the primitive Eodiadema to the complex Cyphosomatidae or the 

 abnormal Echinothuridae. The simplest members of the sub-order can- 

 not be distinguished from the Saleniidae by ambulacral structure alone, 



but they are clearly separated by 

 the absence in this group of any 

 suranal plate. 



FAMILY 1. ORTHOPSIDAE. 

 Diaderuina with the ambulacral 

 plates all simple primaries, and 

 the pore-pairs in a simple series. 

 This interesting family includes 

 1 FIG. xxn. 2 a series of simple Echinoids, 



Aspidodiadema tonsum (after A. Agassiz). 1, from which form the beginning of the 

 the side ; 2, from above showing the apical system , jv, T\-~J ~- T*. 



composed of a single ring of ten plates. sub - order Diademma. It in- 



cludes seven genera ranging from 



the Middle Lias to the present day, viz. Eodiadema, Dune. ; Archaeo- 

 diadema, Greg. ; Orthopsis, Cott. ; Gymnodiadema, Lor. ; Peronia, Dune. ; 

 Echinopsis, L. Ag. ; Aspidodiadema, A. Ag. The only living genus is 

 Aspidodiadema, a deep-sea form dredged by the Challenger. It has 

 been made the type of a special family by Duncan, but it seems to 



