156 



ECHINODERMA. EDRIOASTEROIDEA 



CLASS IV. EDRIOASTEROIDEA 



The calyx in the Edrioasteroids (fig. 68 A) is usually 

 composed of a large number of irregular plates, and in 

 most cases is flattened and more or less circular in outline ; 

 it is attached to some foreign body by the under part — a 

 stem being very rarely present. The mouth is at the 

 centre of the upper surface (ps), and is covered by plates ; 

 from it five ambulacra extend outwards over the upper 



arnb 



Fig. 68. Edrioaster bigsbyi, Ordovician of Canada. (From Bather.) 



A, oral surface, amb, covering-plates over the anterior and left-anterior 



ambulacral grooves, but removed from the other grooves ; ad, floor- 

 plates of ambulacral grooves ; p, pores between floor-plates ; ps, peri- 

 stome, the greater part of which is roofed by enlarged covering- 

 plates ; ia, interambulacrum ; M, madreporite ; As, anus. Natural 

 size. 



B, section across the same specimen through the right anterior ambu- 



lacrum and the left posterior interambulacrum. Lettering as in A. 

 /, frame of stouter plates ; m, membrane with overlapping plates 

 thrown into five lobes (I). Natural size. 



C, section across an ambulacrum with covering-plates (amb) over the 



groove (vg). Enlarged. 



surface of the calyx, and sometimes over part of the lower 

 surface also. The ambulacra do not branch, but are 



