REGULAR EUECHINOIDS. 



333 



In many genera of the EcJiinid<z each ambulacral plate carries three 

 pairs of pores, and types exhibiting this character appear as early as the 

 Jurassic period. One of the leading genera thus characterised is Stoni- 

 echinus (fig. 256), which is found in both the Jurassic and Cretaceous 

 deposits. An allied Jurassic genus is Polycypliusj while Salmacis and 

 Eclmuis proper begin in the Eocene Tertiary, and are represented at the 

 present day by living species. On the other hand, in other genera of the 

 EchinidcB each ambulacral plate carries more than three pairs of ambu- 

 lacral pores. The forms of this group are comparatively modern, a few 



Midlife- 



mm w 







Fig. 256. — Stomechinus lineatus, from the Jurassic rocks (Coral Rag) of Germany. «, A 

 specimen viewed in profile, of the natural size ; b, Part of the oral region of the same. (After 

 Zittel.) 



types {Phymechinus, Pedinofisis, &c.) occurring in the Upper Jurassic or 

 Cretaceous rocks, but the majority being Tertiary or Recent. The two 

 most important members of this section are Sphcerechinus and Strongylo- 

 ce?itrotus, both of which begin in the Pliocene Tertiary, and are repre- 

 sented by living species. 



Irregular Euechinoids. 



The sub-order of the Irregulares (Echinoidea exocyclica) includes 

 all those Euechinoids in which the test is "irregular," the anus being 

 excentric, and not connected with the genital disc. The mouth is 

 placed on the lower surface, and may be central or excentric in 

 position, while a masticatory apparatus may or may not be present. 

 The test is bilaterally symmetrical, commonly of an oblong, penta- 

 gonal, heart-shaped, or discoidal figure (as in the common " Heart- 

 urchins " and " Cake - urchins "). There are commonly only four 

 genital plates in the apical disc, and the ambulacra may be either 

 simple or petaloid. 



The Irregular Euechinoids have been divided into two groups, 

 according as they possess a masticatory apparatus (Gnathostomatd), 

 or are without teeth (Atelostomald) ; but these two groups are very 

 closely connected, and it seems that too much weight has been 



