ORISKANY SANDSTONE. 417 



Strophoiuena rugosa, var. veiitricosa, 



Plate XCIV. Fig. 2 e, f, & 3. 

 Strophonuna depressa, Yar . vetUricosa : 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. Ventral 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 sublrnear, 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 Strophomma rugosa, though its internal muscular 

 attachments often deviate considerably from tliose 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 eleration 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. 



[ PALiEONTOLOOY III.] 53 



