TRENTON PERIOD. 71 



verse diameter being greater than the longitudinal ; dorso-ventral diameter 

 comparatively small in young shells, but it increases with age, so that some 

 old shells are very ventricose ; hinge-line sometimes a little less than the 

 greatest width of the shell, but generally about equaling it ; usually, the 

 antero-lateral margins are regularly rounded and the front a little emargi- 

 nate ; postero-lateral margins generally almost straight from about the mid- 

 length of the shell to the extremities of the hinge-line, with which they 

 form more or less distinct angles. 



Dorsal valve more convex than the ventral, even in the young, and it 

 increases in convexity with age more than the ventral valve does ; greatest 

 convexity at or behind the middle, an indistinctly- defined longitudinal 

 depression, or mesial sinus, is observable in. many shells, but in some it is 

 absent, even at the front margin, and is represented only by a slight flat- 

 tening of the valve in the visceral region ; umbo prominent ; beak abruptly 

 incurved ; area moderately wide in the middle, narrowing to acute points 

 at the extremities of the hinge-line, concave transversely ; foramen broad 

 at base, triangular. 



Ventral valve broadly convex, convexity greatest near the beak ; sides 

 very slightly convex transversely ; mesial sinus scarcely defined on the 

 posterior half of the valve, but in front it consists of a broad, usually very 

 shallow depression, which becomes obsolete about the middle of the valve ; 

 beak moderately prominent.; area a little wider than that of the dorsal valve, 

 and, like that area, it ends in acute angles at the extremities of the hinge- 

 line ; this area less arcuate than the other, sometimes arching a little back- 

 ward, sometimes vertical with the plane of the valve, and sometimes 

 inclining a little forward ; foramen triangular, a little higher than wide, 

 extending to the apex of the beak. 



Surface of both valves marked by distinct, prominent, radiating striae, 

 which increase both by implantation and bifm'cation, and are crossed by a 

 few concentric lines of growth. 



Mature specimens average about two and a half centimeters in length 

 and thi-ee centimeters in breadth. The more gibbous specimens of that 

 size sometimes reach nearly two and a half centimeters in dorso-venti-al 

 diameter. 



