134 
PALAEONTOLOGY OP NEW-YOEK. 
The outline of the valve presents a very regular convexity, while it is 
abruptly depressed towards the cardinal extremities, which are flat¬ 
tened and a little deflected to the ventral side. 
The dorsal valve is variably concave, sometimes following nearly the 
contour of the ventral valve, but often very moderately concave or 
nearly flat in the middle and upper part, and more suddenly deflected 
towards the front and lateral margins, flattened at the cardinal extre¬ 
mities, and a little concave just below the hinge-line. 
The surface is marked by numerous closely arranged slender subequal 
striae, which are bifurcated or increased by intercalation, and are con¬ 
tinued on the cardinal extremities to within a little distance of the 
hinge-line; beyond which, the surface is marked by lamellose con¬ 
centric striae. In well-preserved surfaces, the radiating striae are crossed 
by undulating concentric striae; but in the greater number of speci¬ 
mens, these are not preserved, and the radiating striae have a fibrous 
appearance.* 
The cardinal margin of the ventral valve is furnished with five, six or 
seven oblique tubular spines on each side of the apex, though usually 
only three or four are visible. The ventral area is usually narrow, subli- 
near, though often perceptibly triangular; the foramen is of moderate 
size, partially closed by a convex pseudo-deltidium, and the lower part 
occupied by the cardinal process. The dorsal area is linear, often more 
than half as wide as the ventral area, with a triangular space in the 
middle occupied by the cardinal process. 
The interior of the ventral valve shows strong, very diverging dental 
lamellae, a narrow median ridge with narrow oval occlusor muscular 
impressions, while the divaricator muscular impressions are wide and 
spreading; sometimes lobed or striate, and not very strongly limited 
on the distal margins. Outside of these, are the broader vascular areas; 
beyond which, and sometimes within their limits, the surface is strongly 
pustulose. The casts show the reverse of these features in a median 
depression and widely diverging dental impressions; the surface being 
* Probably from a partial solution of the surface by iron pyrites. 
