352 MACTRIDJE. 



are, however, closely grooved in a concentric direction with 

 abruptly terminating furrows. The valves are hut mo- 

 derately ventricose, and the convexity is evenly enough 

 diffused, not being confined to the umbones as in certain 

 other Mactr^e. The ventral margin is simply convex, and 

 rises equally at either extremity. The dorsal edges are 

 more or less convex, and the declination nearly equal on 

 both sides ; the curve of the anterior is, however, greater 

 than that of the higher posterior one ; and the degree of 

 slope varies from slightly to decidedly declining, according 

 to the greater or lesser disposition toward angularity in the 

 example. The dorsal areas are but little flattened, the 

 hinder is the more so ; the adjacent umbonal ridge is evi- 

 dent, but is not a prominent character. The sides are 

 very nearly, if not quite equal ; if either, the anterior one 

 is very slightly the longer, and is attenuatedly rounded 

 at its extremity, the most elevated portion of the arch not 

 beino- subcentral, but below the middle of the shell ; the 

 posterior termination is almost rounded, being only very 

 bluntly subangulated towards the lower margin. The lips 

 of the dorsal slopes are elevated or pouting, and not in- 

 wardly inclined ; the umbones are not prominent, and the 

 beaks, which are small and in no way remarkable, lean a 

 little forward. 



The internal surface is of a polished but not a snowy 

 white ; the scars, which are profound, are moderate in size, 

 the pallial sinus is small. The teeth are strong, the lateral 

 ones of moderate length and transversely grooved, both of 

 them approximate, but the hinder one the more closely so, 

 and subtruncated near the V-shaped primary denticle ; 

 their upper edges are but little convex, and often nearly 

 straight, so as to appear subtruncated above : the cartilage 

 is not very large. 



