SCHIZOPHORIA 161 



dinal area obsolete, the delthyrium rather large, about as wide or wider 

 than high. Internally the hinge-teeth are large and strong, the muscular 

 impressions large, flabellate, deeply impressed and usually extending two- 

 thirds the length of the valve from the beak, the central or adductor im- 

 pressions are situated upon a narrow, raised, median ridge ; the anterior 

 margin of the valve finely crenulated internally. 



Brachial valve more rotund than the pedicle, with its greatest convex- 

 ity near the middle or a little posterior to the middle, from which point 

 the surface curves to the margins in all directions, the curvature being a 

 little more abrupt to the postero-lateral margins ; the mesial fold scarcely 

 differentiated from the general convexity of the valve except sometimes 

 near the anterior margin ; the beak pointed and slightly incurved. In- 

 ternally the cardinal process is large and strong, sometimes extending 

 anteriorly from the beak for one-fourth the length of the valve ; the mus- 

 cular scars less deeply impressed than those of the opposite valve, divided 

 along the median line by a narrow, elevated ridge ; the anterior margin of 

 the valve finely crenulate interiorly. 



The surface of both valves marked by exceedingly fine radiating costse, 

 about three of which occupy the space of one millimeter at the front 

 margin of a mature shell, these costee are commonly worn away from the 

 greater portion of the valves so that their surface appears to be nearly or 

 quite smooth, but they may usually be detected in the mesial sinus near 

 the front of the pedicle valve where the surface is more protected than 

 elsewhere. 



Remarks. This species is a well-marked form, characterized by its 

 small size, its short hinge-line and especially by the obsolete cardinal area, 

 the marked flattening of the posterior portion of the pedicle valve and 

 the more deeply depressed sinus near the front of the same valve than is 

 common in the genus. The size of the species rarely exceeds that of the 

 larger individual whose dimensions are given above, although the largest 

 specimen observed has a length of 17 mm. and a width of 17.8 mm. The 

 average size of the species is not far from that of the smaller example 

 whose dimensions are given. 



Horizon. Keokuk limestone, Warsaw formation and Salem limestone. 



Genus SCHIZOPHORIA King 



Description. Shell subcircular or subelliptical in outline, resupinate 

 or with the valves subequally convex. Hinge-line shorter than the great- 

 est width of the shell, the cardinal areas rather low. The pedicle valve 

 with short dental plates which are continued as rather sharp ridges along 

 the floor of the valve bounding the muscular area. The muscular scars 

 of the pedicle valve less than half the length of the valve, obcordate or 

 subovate in outline, more or less deeply impressed in the substance of the 



6 



