78 CONTRIBUTIONS 



gin. The anterior part is sometimes entirely straight from 

 the beak to the anterior basal margin. 



P. ellipsis. Plate 3. Fig. 56. 



Description. Shell transversely elliptical, compressed, 

 inequilateral, concentrically and minutely striate, with 

 small longitudinal rugose ribs, more strongly marked on 

 the posterior and anterior parts ; beaks rather elevated, re- 

 curved ; hinge teeth very small, numerous and scarcely 

 interrupted in the middle ; cicatrices impressed ; cavity of 

 the shell shallow ; margin entire. 



Diam. .2, Length 7-20ths, Breadth 9-20ths, of an inch. 



Observations. In some of its characters the ellipsis close- 

 ly resembles the minor, but is eminently distinguished by 

 the absence of marginal crenulations. It is more transverse 

 and has smaller dorsal teeth. Immediately under the beak 

 is a fosset-like impression, which may have been the seat 

 of the ligament. In this it has some resemblance to the 

 nucula; but is placed above the arched row of teeth. 



P. obliqua. Plate 3. Fig. 57. 



Description. Shell oblique, somewhat inflated, very in- 

 equilateral, straight at dorsal margin, with minute longitu- 

 dinal ribs, concentrically folded ; folds beautifully sculp- 

 tured ; beaks rather elevated, pointed ; hinge teeth few, 

 small, placed on nearly a straight plate, smaller in the 

 middle ; cicatrices slightly impressed ; cavity of the shell 



