278 PALEONTOLOGY 



Chief genera of (2) are Echinocorys \_Ananchytes~] (Fig. 81, 

 Cret), oval, with flat base, periproct sub-marginal; Holaster 

 (Fig. 82, Cret.-Mio.), heart-shaped with anterior groove, 

 slight separation of bivium and trivium ; M ' waster (Cret.- 

 Mio.), already described ; Hemiaster (Cret.-Rec.) and 

 Schizaster (Eoc.-Rec.) differ 'from Micraster in not having 

 a sub-anal fascicle, but have a peripetalous fascicle running 

 round the petaloid ambs, which are deeply sunk, especially 

 in Schizaster, which has also a lateral fascicle branching 

 off from the main fascicle on each side. 



The Echinoidea are by far the most important, geo- 

 logically, of the Eleutherozoa. Of the Stelleroidea it is 



a 



FIG. 82. HOLASTER SUBGLOBOSUS. LESKE, CENOMANIAN. 



a, Aboral view. (Xj.) b, Apical disc. (Xs-) (After Wright.) 



only necessary to say that they are known from the 

 Cambrian upwards, and where they do occur it is fre- 

 quently in "starfish-beds" (e.g., where representatives of 

 one or a few species are extremely abundant), yet between 

 such beds there are enormous thicknesses of rock desti- 

 tute of any trace of them. A knowledge of them is, 

 therefore, far less needed by the geologist than that of 

 echinoids or crinoids. 



The Holcthurians have their skeleton reduced to 

 microscopic spicules, so that special search has to be 

 made for them. They are found, here and there, from 

 the Cambrian upwards. 



