70 University Geological Survey of Kansas. 



Larger or ventral valve moderately convex, the greatest con- 

 vexity being in the central region, or rather on each side of it, 

 as there is usually a broad, shallow, mesial depression ; ears 

 and lateral regions compressed ; front somewhat straightened 

 along the middle ; beak small, rather compressed, a little 

 arched, and scarcely projecting beyond the cardinal margin, 

 which is provided with from seven to eleven oblique spines on 

 each side of the beak ; area rather narrow, ranging nearly 

 parallel with the general plane of the valves, its fissure broad, 

 partly closed by the arching deltidium ; hinge teeth well devel- 

 oped, compressed, and minutely striated; interior with impres- 

 sions of cardinal muscles subovate, diverging, attenuate above ; 

 adductor muscular scars small, narrow-subelliptical ; mesial 

 ridge prominent near the beak, much lower, and nearly extend- 

 ing forward to the central region ; most of the interior occupied 

 by granules, which are largest and most crowded on a narrow 

 space around and near the front and lateral margins ; but 

 around the immediate margin they are much smaller, and ar- 

 ranged in distinct radiating rows. Dorsal or smaller valve 

 following nearly the curve of the other, the beak and central 

 regions being concave, and the ears flat; area well developed, 

 but narrower than in the other valve ; bifid cardinal process 

 and mesial prominence, nearly or quite closing the fissure of 

 the other valve. From the base of this process there are ex- 

 tending, on the inner side of the valve five radiating ridges, 

 two of which pass obliquely outward, forming the inner mar- 

 gins of the dental sockets, while a third mesial one extends at 

 right angles to the hinge, a little more than half way to the 

 front ; the other two are much shorter, oblique, and occupy in- 

 termediate positions between the middle and two latter ones ; 

 granules of the interior as in the other valve. Surface of both 

 valves ornamented with a few subimbricating marks of growth, 

 crossed by very fine, obscure, regularly and closely arranged 

 radiating striae, of which about 150 can be counted around the 

 free border of a large individual, where about eight or nine of 

 them may be counted in the space of one line." 



