BLASTOIDEA. 



311 



period. They make their appearance in the Silurian, and reach their 

 richest development in the Carboniferous. As at present constituted 

 the class is a well denned one.* They approach the Cystids through the 

 Diploporida. The calyx is somewhat ovoid and either has a short stalk 



FIG. 223. Analysis of calyx of Pentremites 

 ftorealis (from Zittel). Aboral view, the 

 posterior deltoid is downwards in the 

 figure, b basals ; r radials; ir deltoids 

 (interradial). 



FIG. 224. Apical region of Eleutherocrinus 

 Cassedayi (after Etheridge and Car- 

 penter), a- y axis passing through the 

 anal interradius and the opposite radius 

 (III) ; II-V four of the radii. 



or is without a stalk. It consists of three rows of plates (Fig. 223) ; three 

 basals, two of which are larger than the third as though composed of 

 two pieces fused ; five radials (r) each of which is forked at its radial end ; 

 and five interradial deltoids (ir), which surround the peristome. The 

 spaces between the forks of the radials and between the deltoids are 

 1 



aunta. 



FIG. 225. Cadaster trilobatus (after Bather). 1 oral surface of young form x 6. 2 

 slightly restored section through a radius. In 1 the central parts of the deltoids are 

 prominent, amb ambulacrum ; As anus ; br pinnule (brachiole) ; c.p covering plates of 

 ambulacra! groove ; h hydrospire slits ; the hydrospires of the anal interradius are 

 imperfect in this genus ; L lancet plate, containing canal ; mouth ; R radial ; s.p side 

 plate ; A deltoid. 



occupied by the ambulacra (Fig. 225 amb}. The ambulacra are petaloid 

 and are traversed by a median ambulacral groove. The floor of this 



* Bather has established a subclass of Blastoids to which he applies the 

 name Protoblastoidea to include Asteroblastus and Blastoidocrinus (p. 310). 

 These genera, however, are without hydrospires and possess diplopores. 



