 •9 PALEONTOLOGY OP NEW-YORK. 



Ventral valve regularly convex, often gibbous; greatest elevation 

 nearly central, and sometimes subangulated along the middle : umbo 

 small and prominent, with the apex slightly incurved and extending 

 beyond the plane of the area ; surface a little concave towards the car- 

 dinal angles, which are slightly deflected. Dorsal valve moderately 

 concave, rarely following the convexity of the opposite valve ; some- 

 times an undefined median depression extends from beneath the apex 

 to the front of the shell. 



Area of ventral valve variable, usually of moderate width, from tItf to 

 -^ of an inch wide in the centre, having a low triangular outline, con- 

 cave in the middle and for a considerable distance on each side of the 

 beak, strongly striated transversely and more faintly longitudinally, 

 sometimes marked along the middle by a subangular elevation ; inner 

 margin crenulated for nearly its entire length. There is no foramen, 

 but sometimes a smooth triangular space beneath the beak. Dorsal 

 area narrow and usually linear, sometimes wider and sometimes nar- 

 rower in the middle, and the margin for a short space free from crenu- 

 lations. The planes of the two areas are inclined so as sometimes to give 

 less than a right angle between them, but generally a greater angle, 

 and along the middle the two are often nearly in the same plane. 



Surface marked by numerous T^ovvded strife, about nine or ten of which 

 are much stronger and more elevated on the umbo of the ventral 

 valve, with finer ones coming in between and on either side : striae 

 frequently increasing by intercalation and bifurcation, until they 

 become very numerous and much finer at the margin. On the 

 dorsal valve, the stria3 are similar to those of the ventral valve. In 

 well-preserved specimens, fine concentric striae cover the entire sur- 

 face, but the greater number of specimens do not preserve these mark- 

 ings. The coarser striae are sometimes seen separated on the middle 

 of the shell, each one presenting the appearance of a fascicle of striae, 

 which, spreading, cover the lower part of the shell with extremely 

 fine crowded striae. 



