ATRYPA OF THE SCHOHARIE GRIT. 
315 
Atrypa impressa. 
PLATE LI. 
Atrypa impressa: Hall, 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 
angular 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. 
The 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. 
