ORISKANY SANDSTONE. 
417 
Stropliomciia rugosa, var. veiitricosa. 
Plate XCIY. Fig. 2 e , f , & 3. 
Strophomena depressa, var. ventricosa : Hall, Regents’Report for 1856, p. 55; Pal. Foss. 1857, p. 15. 
Shell transversely oblong, somewhat semioval; length and breadth 
sometimes nearly equal; front often straight in the middle : hinge-line 
equal to the greatest width of the shell. Yentral valve extremely ven- 
tricose, scarcely geniculate in front. Dorsal valve forming an inclined 
plane from the hinge towards the front, near which it is abruptly 
deflected, giving the valve a deep concavity; area sublinear, longitu¬ 
dinally striate : interior distinctly granulose; muscular attachments 
strongly marked ; lateral margins contracted, so as to leave small 
auricular extensions at the cardinal angles. 
Internal casts, with fragments of separate valves of this shell have been seen : 
its general aspect is like that of Strophomena rugosa , though its internal muscular 
attachments often deviate considerably from those of well-marked specimens of that 
species. The ventral valve is also more regularly arched and gibbous in outline 
than is usual in S. rugosa ; and it has not the abrupt geniculation in front, so 
characteristic of that shell. 
The casts of the interior of the ventral valve are almost regularly ventricose, a 
little more abruptly bent in front; the muscular impressions are well marked, and 
deeply divided longitudinally in the middle; the median ridge being broad at the 
base, with a narrow thin crest, as shown both in the casts and in the interior of the 
valves. The dental lamellae are produced in a thin elevated crest which surrounds 
the muscular area, except a narrow space at the base. 
In the casts of the dorsal valve, the muscular imprints are strongly marked with 
two large pits for the double cardinal process. The casts taken from these impressions 
preserve the same general features as those of the S. rugosa from the limestones of 
the Lower Helderberg group, but are more elongated, and more strongly defined 
in the elevation of the parts above the general surface of the interior of the shell. 
The interior of this valve, moreover, does not present the marked elevation which 
limits the vascular area, or that portion just within the geniculation of the shell. 
A single dorsal valve of this form has a length of one inch and three-fourths, with 
a width of two inches. 
[ Paleontology III.] 
53 
