ECHINODERMA. EDRIOASTEROIDEA 157 



frequently curved. The ambulacral grooves are covered 

 by two rows of alternating plates (amb), similar to the 

 covering-plates of crinoids. In Edrioaster and its near 

 allies the floor of each groove is formed of special plates 

 (ad), between or at the outer margins of which are pores 

 (p) which may indicate the existence of tube-feet. Neither 

 brachioles nor arms are developed in connexion with the 

 ambulacra. The anus, which is covered by a pyramid of 

 plates, as in the Cystidea, is on the upper surface — in the 

 area between the two posterior ambulacra (As). The calyx 

 was more or less flexible in some cases ; and frequently 

 around its border on the upper surface (but sometimes 

 on the lower, fig. 68 B, f) there is a series of larger 

 marginal plates, forming a framework, which, in combination 

 with the five conspicuous ambulacra, gives the upper 

 surface something of the appearance of a star-fish in which 

 the rays are not prolonged. 



The Edrioasteroids include only a few genera, and have 

 usually been regarded as Cystidea, but differ in the 

 absence of brachioles, and in the occurrence of pores 

 between the flooring-plates, suggestive of the presence of 

 an internal radial water vessel with tube-feet. 



Distribution of the Edrioasteroidea 



The Class ranges from the Cambrian to the Carbon- 

 iferous, and is best represented in the Ordovician. The 

 principal genera are : — Stromatocystis in the Cambrian ; 

 Cyathocystis, Edrioaster and Steganoblastus in the Ordo- 

 vician ; and Agelacrinus and Lepidodiscus, ranging from 

 the Ordovician to the Carboniferous. 



