ATRYPA OP THE SCHOHARIE GRIT. 315 



Atrjpa iiiipressa. 



PLATE LI. 



Atrypa impreiia: Hail, Tenth Report on the State Cabinet, p. 122 (Pal. Fossils, p. 82). 1857. 



Shell somewhat ovoid, often extremely gibbous ; length and breadth nearly 



equal. 

 Ventral valve depressed convex in the lower part, becoming gibbous above, 



more prominent along the middle, but without any defined elevation, and 



without sinus, except in very rare examples where the extreme front 



of the shell is bent upwards in the middle : beak closely incurved over 



the umbo of the opposite valve. 

 Dorsal valve the larger, very convex, and becoming extremely gibbous in 



full grown and old shells. A more or less distinct mesial elevation marks 

 ' the lower half of its length : this is often flattened, giving an obtusely 



ang"ular outline, and the centre is marked by a narrow depression : the 



front is squarely truncate or impressed. 

 Surface marked by numerous radiating striae or costae, which are frequently 



bifurcated, and crossed by numerous concentric lamellose lines of growth. 



Thfe casts of the interior, which are more abundant than any other con- 

 dition of the fossil in the Schoharie grit, present considerable variety of 

 aspect. The cast of the ventral valve is usually a little convex, or nearly 

 flat outside of the muscular area, and often a little more prominent in the 

 middle than at the sides. The muscular area is broad flabelliform, wider 

 than long, depressed and smooth on each side above, with a central promi- 

 nence for the attachment of the adductor muscles, from which there is 

 usually a sharp carina extending to the beak : there is also a less conspicuous 

 ridge on each side, extending obliquely from the apex to the base of the 

 cavity left by the teeth. Outside of the muscular area the surface is 

 strongly papillose, and sometimes marked by vascular impressions. 



