306 THE ECHINOIDEA 



SUB-CLASS 2. RKGULARIA ECTOBRANCHIATA. 



Echinoidea with mouth and anus at opposite poles. Endocyclic, with 

 external gills. 



ORDER 1. Diademoida, Duncan. 



Echinoidea Regularia Ectobranchiata in which the mouth and anus 

 are both central and opposite. The anus opens in the centre of the apical 

 system (which may however be rudimentary). The external branchiae 

 pass out through the buccal clefts. A dental apparatus is present. 

 There are no interambulacral plates on the peristomial membrane. The 

 ambulacral plates are generally compound. 



This order marks a great advance on any of those previously 

 denned. The ambulacral plates in some forms remain as simple 

 primaries, but in the majority they unite into compound plates, different 

 from anything met with in the preceding groups. At the same time 

 external gills appear, with or without internal gills, and none of the 

 interambulacral plates occur on the peristomial membrane. The order is 

 accepted practically as defined by Duncan, 1 but his method of division is 

 not followed. 



SUB-ORDER 1. CALYCINA. 



Diademoida in which the apical system is very large and includes 

 one or more supplementary suranal plates. 



As we have seen in the description of Tiarechinus and Lysechinus, 

 these small forms gained strength by the development of a series of 

 large apical plates. In one group of the Diademoida the same result 

 is obtained by the incorporation of one or more "suranal plates" 

 in the apical system. Like Tiarechinus, 

 the Echinoids in which this feature first 

 appeared are very small. The character has 

 persisted from the Trias to the present time, 

 but the Echinoids in which it occurs are never 

 large. 



FAMILY 1. SALENIIDAE. Calycina in 

 which the ambulacral plates are simple 

 primaries, and there is one large suranal 

 plate in the apical system. Genera Salenia, 

 Gray ; Heterosalenia, Cott. ; Peltastes, L. Ag. 

 FIG. xix. ( Fig> XVI. 5 ; Fig. XIX.) ; Goniopkanu, L. Ag. ; 



Saknia varispina. Abactinal Saueria Ebert 

 surface. 



FAMILY 2. ACROSALENIIDAE. Calycina in 



which there are one or more suranal plates in the apical system ; the 

 ambulacral plates are simple primaries near the apical system, but com- 

 pound, with demi-plates near the peristome. Genus Acrosalenia, L. Ag. 



1 The periguathic girdle is not ahvays continuous as stated in his diagnosis. 

 Duncan (24), p. 24. 



