460 CYPRINID^. 



resembles that of sulcata. The valves are usually com- 

 pressed (more rarely convex), not heavy, and only mo- 

 derately solid, opaque, and covered with a rufous or, less 

 commonly, olivaceous brown cuticle, becoming fulvous upon 

 the umbones, beneath which the surface, as well as the in- 

 terior, is of a more or less squalid white. The more or 

 less depressed and rounded ribs, which traverse the shell 

 concentrically, and are usually at least as broad as their 

 interstices, yet not very closely arranged, generally grow 

 faint or utterly vanish towards the ventral margin, and 

 always cease, excepting those in immediate proximity to 

 the beaks, beyond the site of the obsolete umbonal ridge. 

 The concentric striulse, which are only occasionally met 

 with in sulcata^ are here permanently characteristic, and 

 pervade the ribs as well as the interstices. The ven- 

 tral margin is convex or subarcuated ; the front dorsal edge 

 concave, and decidedly sloping, but varying in intensity of 

 curve and declination ; the slope of the hinder dorsal edge, 

 whose convexity is by no means strong, is trifling or very 

 moderate. The beaks which are inclined, tolerably acute, 

 and not very prominent, are situated at about one-third the 

 distance from the rounded anterior extremity to the ob- 

 tusely and not broadly, biangulated termination of the pos- 

 terior side. The lunule, lozenge, hinge, and ligament, are 

 similar to those of the preceding species ; the tAvo first are 

 perhaps more deeply excavated, and the last darker and 

 possibly larger. The inner surface of the margins is en- 

 tirely free from crenulation, even in the most aged ex- 

 amples. The muscular impressions are nearly double the 

 size of those of sulcata. The dimensions of rather a 

 large specimen are an inch and a quarter in length, by 

 eight-ninths of an inch in breadth. 



This shell appears to be almost wholly confined on our 



