ECHINOIDEA. 239 



Cretaceous. It is easily recognised by the great size of the 

 anal aperture, which is oblong or pyriform in shape, and is 

 placed upon the upper surface of the test. The genus Hybo- 

 (fig. 134), again, is an aberrant type, since the mouth, 



Fig. 134,Hy'bnclypns gilberulus, viewed from above, from one side, and from below. 



Jurassic. 



though inferior, is not central ; the opening of the anus is 

 placed in a longitudinal dorsal valley ; and the posterior two 

 ambulacral areas are disjoined from the anterior three, con- 

 verging (as in the Collyritidce) to two distinct points upon 

 the upper surface of the test. Of the remaining genera of 

 this family the two most important are Galerites (fig. 119) 

 and Discoidea (fig. 132), constituting two nearly related types 

 which are widely distributed in the Cretaceous rocks. Holec- 

 typus (fig. 136, A), lastly, is closely allied to Discoidea, and 

 is principally confined to the Jurassic period. 



In the small and singular family of the Collyritidce (fig. 

 135), the test is usually ovate or heart-shaped; the ambu- 

 lacral areas are narrow, and are " disjunct," meeting at two 



Fig. 135. Collyrites (Disaster) Ewle$i t viewed from above, from one side, and from below. 



Jurassic. 



more or less remote points upon the upper surface ; and the 

 tubercles are small, perforated, and crenulated. The mouth 

 is inferior and excentric ; the anus is supra-marginal ; and 

 the elements of the apical disc are detached. This family is 



