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PALAEONTOLOGY OP NEW-YOEK. 
Yentral 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 yf o to 
t&o 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 crowded striae, 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 striae 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. 
