238 



ANXULOJDA. 



pentagonal, heart-shaped, or discoidal figure (as in the com- 

 mon " Heart -urchins" and " Cake -urchins ") ; the anus is 

 situated outside the apical disc, usually marginal or submar- 

 ginal in position ; there are mostly only four genital plates 

 in the apical disc ; and the mouth is mostly destitute of 



Fig. 132. Discoidea cylindrica, an "irregular" Echinoid. The right-hand figure shows 

 the summit of the shell, with the genital disc. The left-hand figure shows the base of the 

 shell, on which are situated both the mouth and anus. Cretaceous. 



a masticatory apparatus. The Irregular Echinoids may be 

 divided into the families of the Echinoconidce, Collyritidm, 

 Echinonidce, Echinobrissidce, Echinolampadce, Clypeastridw, 

 Ananchytidm, and Spatangidoe. 



In the Echinoconidce the test is usually circular or pen- 

 tagonal, the ambulacral areas narrow, and the plates of both 



Fig. 133. Pygaster truncatus, viewed from above, from behind, and from one side. 

 Cretaceous. 



areas carrying small, crenulated, and perforated tubercles. 

 The mouth is inferior and central in position, toothed ; the 

 vent is on the upper surface, marginal, or inferior ; the apical 

 disc central, superior, and composed of the normal ten 

 plates. The spines are short and awl-shaped. 



All the members of this family are found in the Oolitic and 

 Cretaceous rocks. The genus Pygaster (fig. 133) commences 

 in the lowest Jurassic deposits, and extends upwards into the 



