240 ANNULOIDA. 



wholly confined to the Jurassic and Cretaceous rocks, and 

 by far the most important genus contained in it is Collyritcs 

 (Disaster or Dysaster) itself. 



In the small family of the Echinonidce the test is oval, 

 the ambulacral zones meeting at the apical disc, and the 

 tubercles neither perforated nor crenulated ; this last char- 

 acter separating the family from the Echinoconidce. There 

 are only four genital plates in the apical disc ; the mouth 

 is inferior, central, and toothless ; and the anus is usually 

 supra-marginal. The only fossil genus is the Pyrina of the 

 Cretaceous rocks. 



In the family of the Echindbrissidce (Cassidulidce in part) 

 the shape of the test is variable ; the tubercles are small, 

 imperforate, un crenulated, and surrounded by sunken areolce; 

 the spines are short and slender ; and the ambulacral areas 

 are petaloidal, lanceolate above, or sub - petaloidal. The 

 mouth is inferior, sub-central, and edentulous ; and the anus 

 (fig. 136, B) is dorsal, opening in a valley, or supra-marginal. 



Fig. 136. A, Holectypus Jiemisphericus, viewed from above Jurassic (after Edward Forbes) ; 

 B, Echindbrisms clunicularis, viewed from above Jurassic (after Wright). 



Besides the living Nudeolites, the family comprises a large 

 number of Jurassic, Cretaceous, and Tertiary Urchins, be- 

 longing to the genera Echinobrissus (fig. 136, B), Clypeus, 

 Catopygus, Pygaulus, &c. 



In the family of the Echinolampadce (Cassidulidce in part) 

 we have Urchins in many respects agreeing with the Echino- 

 Irissidce, especially in their petaloidal or sub-petaloidal am- 

 bulacra, but having the anus marginal, infra-marginal, or 

 supra-marginal, in no case opening into a dorsal sulcus. 



