INVERTEBRATES. 335 



ORDER AGELACRINOIDEA, 



FAMILY AGELACRINmZE. 



ECHINODISCUS, n. gen. 



(Ety, echinus, sea urchin; diskoa, a round plate.) 



Body discoid, depressed convex, the smaller plates being found in 

 the border and ambulacra adjacent thereto, and the larger plates 

 occupying the central part of the interradial spaces, none of them 

 imbricating. The border consists of narrow plates elongated in the 

 direction of the circumference, and passing around upon the under 

 side, forming a cylindrical nonsessile rim. Ambulacra five, con- 

 nected near the central area, but not arising from a common point, 

 consisting of narrow, curving angular arches, sharply defined on 

 either side, and composed of numerous interlocking or dove-tailing 

 plates. Mouth or ovarian pyramid central or subcentral. 



This genus is distinguished from Agelacrinus, with which it seems 

 to be most nearly related, by many important characters. First, 

 it is not composed of imbricating plates. Second, the manner of 

 attachment to other objects was different, as in this genus the mar- 

 gin was free. Third, the ambulacra do not arise from a pyramidal 

 elevation or common point, but, one may be said to arise on one 

 side of the mouth, and to connect, by an ambulacral series of plates, 

 with the points, at which two arise, on either side of it, or that two 

 series bifurcate, if it is considered that three series originate on one 

 side of the mouth, in the first instance. Fourth, the mouth or ovarian 

 pyramid is subcentral, while in Agelacrinus it is submarginal. This 

 elevation would seem to be homologous with the mouth in the echi- 

 noids, for below it, within the visceral cavity, there occur several 

 pieces, which were evidently connected with the digestive functions, 

 and therefore homologous with the jaws in the latter order. 



