332 ECHINOCORYS. 



Description. — This fine large Urchin, so very characteristic of the Upper Cretaceous 

 formation, is rounded before and slightly contracted behind the upper surface, is much 

 elevated, inflated, and more or less regularly convex. Sometimes elevated and conical, 

 always slightly carinated in the posterior region. The inferior surface is flat, more or less 

 rounded at the border, and deeply depressed around the mouth, presenting, in the median 

 plane, an elevation which corresponds to the single interambulacral area. The ambulacral 

 summit is nearly central. The ambulacra separated without being disjoined, are entirely 

 similar to one another, and visible from the summit to the mouth ; the poriferous zones 

 are formed of round holes placed obliquely in pairs ; the holes are perforated in the middle 

 of the ambulacral plates (fig. 1 /), a form of structure which is altogether different from 

 other congeneric groups. The pairs of pores are set closely together in the upper part 

 of the zones, more widely apart on the sides, and remote from each other about the 

 ambitus ; near the peristome the pores again approximate, where they are surrounded 

 by a stellate arrangement of the tubercles around the oral aperture (fig. 1 b and e). 



The ambulacral plates are small and narrow in the upper part, and become large and 

 broad in the lower and middle portions of the areas, and they are all perforated near their 

 centres. 



The tubercles developed on the upper surface of the plates of both areas are small 

 and nearly equal in size, and those found at the base are large and prominent. In well- 

 preserved specimens they are seen to be scrobiculated, crenulated, and perforated ; they 

 are small and irregularly placed on the upper surface (fig. 1 «,/), larger and more 

 regularly arranged near the ambitus, and in the middle of the under surface they are much 

 larger and more closely set together, and raised on bosses with crenulated summits 

 (fig. 1 ff, li). The intermediate granulation on the surface of the plates is quite micro- 

 scopic, abundant, and homogeneous, forming regular circles around the tubercles and 

 filling up all the intermediate spaces with minute granules (fig. 1/). 



The apical disc (fig. 1 d) is elongated ; it is formed of four large perforated ovarial 

 plates and five large oculars. Of the two anterior ovarials the right one is the largest, 

 and supports the madreporiform body. The two anterior oculars are larger than the 

 other three, and the orbits in all are marginal ; all the discal elements are closely soldered 

 together and covered with a fine granulation. On flint moulds (fig. 11) the position of 

 the central portion of the apical disc is marked by a space included within an oblong 

 groove. This groove is caused by the impressions of the walls of a solid tube-like body 

 projecting from the inner surface of the apical disc into the cavity of the test and directed 

 backwards. This organ appears to have been the sand canal, and was connected with 

 the madreporiform body, as shown in fig. 11. On flint moulds the pores of the avenues 

 are strongly projecting (fig. 10), where we notice the basal position of the poriferous- 

 zones. 



The small mouth-opening is situated in a deep depression near the anterior border. 

 At the termination of the median elevation the peristome is transversely oval, with a 



