1S4 PALEONTOLOGY OF NEW-YORK. 



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 subequai 

 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 stria3. In well-preserved surfaces, the radiating stria) are crossed 

 by undulating concentric striae ; but in the greater number of speci- 

 mens, these are not preserved, and the radiating strias 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 Trom • partial solution of the surface by iron pyrites. 



