384 



ECHINOZOA. 



attached to the value in classification of this particular character. 

 Moreover, it has not in all cases been certainly ascertained whether 

 or not a masticatory apparatus is actually present. Here, therefore, 

 it will be sufficient to divide the Irregular Euechinoids into the six 

 families of the Conoclypeidce, Clypeastridce, Echinoconidce, Cassidulidce, 

 HolasteridcZ) and Spatangidce. The oldest types of the whole series 

 (Fygaster and Galeropygus) appear in the Lias, and numerous forms 

 are known in the later Mesozoic and Tertiary deposits, while the 

 group is largely represented at the present day. 



Family i. Conoclypeidcz. — The Urchins of this family have a 

 rounded or ovoid, gibbous test, covered with small tubercles and 

 spines. The ambulacra are petaloid, but are continued inferiorly as 

 far as the mouth. There are only four genital plates, and the madre- 

 porite is of large size. The mouth is placed in the centre of the 

 inferior surface, and the anus is submarginal. Teeth are present, 

 and " auricles " are also developed. All the members of this family 



257. — Conoclypus conoideus, from the Eocene rocks of Germany, viewed sideways and 

 from below, reduced in size. (After Zittel.) 



except one are referable to the single genus Conoclypus (fig. 257), the 

 species of which range from the Chalk to the present day, being most 

 abundant in the Eocene Tertiary. 



Fainily 2. Clypeastridce. — In this family the test is usually circular 

 or elliptical, generally depressed, the surface covered with small 

 tubercles surrounded by sunken, ring-like areolce, and carrying hair- 

 like spines. The dorsal portions of the ambulacral zones are wide 

 and petaloid, and the ambulacral pores are confined to the apical 

 "rosette" thus formed. The mouth is inferior, central, and armed 

 with teeth ; and the anus is marginal or infra-marginal. The madre- 

 porite occupies almost the whole of the apical disc, and the genital 

 plates (four or five in number) are only indicated by their pores. 



