IRREGULAR EUECHINOIDS. 



;37 



mately similar, and may be either simple (Echinonidce) or petaloid 

 (Echinolampadaz). Only four genital plates are furnished with per- 



Fig. 261. — Pygaster trtmcatus, viewed from above, from behind, and from one side. 

 Cretaceous. 



forations for the generative ducts. The range of the family is from 

 the Jurassic to the present day inclusive. 



In one series of the Cassidulidtz — sometimes spoken of as the Echino- 

 nidcB — the ambulacra are simple and linear, and the mouth is central or 



£ 



Fig. 262. — Hyboclypiis gibberulus, viewed from above, from one side, and from below. 



Jurassic. 



subcentral. A good example of this is the Jurassic genus Hybodypus 

 (fig. 262), in which the mouth is removed towards the anterior side, the 

 anus is in a longitudinal dorsal valley, and 

 the apical disc is elongated, so that the pos- 

 terior two ambulacra become disjoined from 

 the anterior three. Other examples of this 

 group are the Cretaceous and Eocene genus 

 Pyrina, and the Miocene and Recent genus 

 Edii?ioneus. In another group of the Cas- 

 sidididcE — sometimes spoken of as the Ech- 

 inolampadce — the ambulacra are petaloid, 

 and are generally sunk in the vicinity of 

 the mouth in grooves separated by the 

 swollen interambulacra, giving rise to a 

 peristomial rosette or " floscelle." Some of 

 the members of this group make a near ap- 

 proach to the preceding, being without a con- 

 spicuous oral rosette, and having ambulacra 



of a but very slightly petaloidal form ; and these have sometimes been 

 separated to form a distinct group {Echinobrissidce). Of the many forms 



Fig. 263. — EcJiinobrissus clunicu- 

 ban's, Jurassic, viewed from above. 

 (After Wright.) 



