MYA. 165 



after riinninof a short distance, bends down in an arcuated 

 curve to the ventral, which does not equally rise to meet it ; 

 hence the more or less broad anterior extremity, is well but 

 not symmetrically rounded. The general direction of the 

 hinder dorsal edge is subretuse or straight, and its slope is 

 almost imperceptible. The hinder side, which is barely the 

 shorter, is truncated at its termination, the truncation 

 being almost direct, and not oblique ; the posterior, how- 

 ever, is rather convex than otherwise, and the hinder 

 extremity not absolutely biangulated, but with the angu- 

 lation a little softened off by the terminal convexity of the 

 upper and lower margins. The posterior side is without 

 any manifest umbonal ridge ; but a marked, though broad 

 and undefined, ridge-like elevation very frequently runs 

 anteriorward from the umbones, in a very oblique direction. 

 The umbones are for the most part unequally prominent, 

 the beaks are small, acute, much incurved, and a little in- 

 clined forwards; in front of them there is seen a false lunule, 

 being a kind of amorphous depression, which is continued 

 also beneath the beaks to the opposite side. The whole 

 interior is white and glossy ; the muscular impressions are 

 not large, the posterior one is by far the more profound ; 

 the sinus of the palleal impression is very large, and some- 

 what squared. The hinge consists of a very large, broad, 

 solid, erect, complicated tooth in the left valve, of which 

 the lower surface is convex and simple, but the upper is 

 flattened and subdivided, the central triangular portion 

 forming a very shallow pit for the adhesion of the car- 

 tilage, bounded by an obliquely-radiating, narrow fold in 

 front, with a similar, but much more obscure one, at its 

 posterior termination ; a third well developed fold-like pro- 

 jection is visible at the strengthened posterior extreme. In 

 the right valve there is only a small oblique, fold-like 



