140 



ECHINODERMATA PELMATOZOA 



SUB-KINGDOM III 



Family 2. Hexacrinidae. Wachsmuth and Springer. 



Dorsal cup composed of a monocydic base, five large radials, and an anal plate 

 similar in farm and size to the radials interposed between them. In all other respects 

 constituted like the Platucrinidae. Devonian and Carboniferous. 



Hexacrinus, Austin (Fig. 241). B three, equal; R five, very large and 

 elongate. Tegmen low, composed of relatively large plates, and pierced by an 

 eccentric anal opening. Arms uniserial or biserial; pinnules unusually long. 

 Column round. Devonian ; represented by numerous species in England, 

 France, Belgium, and the Eifel, but by only a few in North America. 



DicJwcrinus, Miinst. Differs from Hexacrinus in having but two hexagonal 

 basals, and an eccentric anal opening. Sub-Carboniferous ; Belgium. Great 

 Britain, and North America. 



Arthrocantha, Williams (Histricrinus, Hinde). Like Hexacrinus, except that 

 the calyx plates are beset with short mobile spines. Devonian ; North 

 America. 



Pterotocrinus, Lyon and Cass. B two, equal. R short, rapidly spreading ; 

 their upper faces excavated for the reception of several orders of brachials, 



FIG. 241. 



Hexacrinus elongatus, Goldf. Devonian ; Pelm, Eifel. a, Calyx seen from one side ; 6, Aspect of summit ; 

 c, Diagram of calyx and arms ; d, e, Column of H. spinosus, Milll. (after L. Schultze). , 



which rest with their lateral edges upon it ; all of these plates taking part in 

 the dorsal cup. IRA less than half the size of the R, and sometimes 

 triangular. Tegmen highly elevated, pyramidal. Orals large, asymmetrical ; 

 their lower angles enclosing five large, radially disposed plates, which are 

 separated from one another by interambulacral pieces. These plates are 

 provided with enormous, leaf-like, club-shaped, or spatulate processes, which 

 project outward conspicuously from between the arms. Anus sub-central. 

 Arms four to each ray ; biserial. Kaskaskia Group ; North America. 



Talarocrinus, "W. and Sp. Intermediate between the preceding and Dicho- 

 crinus; structure of the tegmen resembling the former, that of the dorsal cup 

 the latter. IRA slightly smaller than the R ; its upper end narrower than 



