IRREGULAR EUECHINOIDS. 



385 



The interior of the shell, as a rule, is more or less subdivided by 

 calcareous pillars or septa. The Clypeastrids range from the Chalk 

 to the present day, but the early forms of the group are small. 



The typical Clypeastrids have the test more or less gibbous superiorly, 

 the two most important genera being Echinocyamus and Clypeastcr. The 

 first of these includes small oval Urchins, which range from the Chalk to 

 the present day ; while the latter (fig. 258) comprises large, massive, ellipti- 

 cal or pentagonal types, with a conspicuously developed ambulacral rosette. 



>°ZZ?c-M*. 



yk%°m 



Fig. 258. — Clypeastcr grandijlorus, from the Miocene Tertiary, viewed from above and below, 

 slightly reduced in size. (Copied from Zittel, after Desor.) 



The oldest known species of Clypeaster appear in the Eocene Tertiary, 

 but there are numerous Miocene and Pliocene forms, and the genus is 

 well represented in recent seas. 



In another group of Clypeastrids, exemplified by such genera as Scu- 

 tella and Echinarachnius , the test is so flattened as to become discoid 

 or cake-like. The genus Scutella (fig. 240) is wholly confined to the 

 Eocene and Miocene deposits, while the closely allied types placed 

 under Echinarachnius are still represented in existing seas. 



Family 3. Echiiioconida. — In this family the test is usually cir- 

 cular or subpentagonal, the ambulacral areas being narrow, and 

 running continuously from the apical disc to the peristome (fig. 259). 

 Both the ambulacral and interambulacral areas carry small, crenu- 

 lated, and perforated tubercles, which support short and awl-shaped 

 spines. The mouth is inferior and central, and the excentric anus 

 may be superior in position, but is usually inferior or marginal. 

 There may be only four genital plates in the apical disc. 



vol. 1. 



2 B 



