IRREGULAR EUECHINOIDS. 



3S9 



it. 

 group 



Fig. 266. — Under surface of the test of An an- 

 chytes ovata, showing the position of the mouth 

 and anus. Jurassic. (After Forbes.) 



In another series of the Holasteridce — sometimes spoken of as the 

 Ananchy tides or Echinocoridts — the apical disc may be either elongated 

 or compact, but in either case the 

 five ambulacra all meet 

 The chief genus of this 

 is Ananchytes (fig. 266) itself, in 

 which the test is ovate, and highly 

 convex above, the peristome is ad- 

 vanced forwards and is two-lipped, 

 and the anus is inframarginal. 

 The species of this genus are 

 confined to the Chalk, the com- 

 monest form being the well- 

 known and highly variable A. 

 ovata. The genus Holaster is 

 nearly related to Ananchytes, but 

 the test is sub-cordate, the apical 

 disc is more elongated, and the 

 anus is marginal or supramar- 

 ginal. The species are chiefly 

 Cretaceous, but are found in the 

 Tertiary deposits in Australia. In 

 Cardiaster, again, which is also 

 Cretaceous, there is the addi- 

 tional character of the existence 

 of a " fasciole," which passes be- 

 neath the anus and is continued 

 on the sides of the test. As will 



be seen immediately,' the presence of "fascioles" — that is to say, of cir- 

 cumscribed bands of microscopic granules, occupying definite areas and 

 positions on the test — is highly 

 characteristic of the Spatan- 

 gid<z, towards which Cardiaster 

 thus makes an approach. 



Family 6. Spatangidce. — In 

 the members of this family — 

 often spoken of as " Heart- 

 urchins " — the test is mark- 

 edly bilateral, and is usually 

 conspicuously heart - shaped 

 (fig. 267). The mouth is ad- 

 vanced far forwards on the 

 under side, and the anus is 

 supra-marginal, and is placed 

 in the posterior interambu- 

 lacrum. The ambulacra are 

 petaloid, and the unpaired F [^.26 7 . 

 anterior ambulacrum always 

 differs more or less from the 



others, being usually lodged in a groove or "sulcus" (fig. 267). 

 The mouth is typically bilabiate, but is in some cases five-sided ; 



■Upper surface of Micrastercoranguinum. 

 Cretaceous. (After Forbes.) 



