STROPHODONT^ OF THE UPPER HELDERBERG GROUP. 87 



Figure 12 represents a young individual, and figure 13 is twice enlarged. 



The figures 18 & 19 are ventral and profile views of a large individual, and the fasciculate 



striae upon the inner surface of the shell are shown in figure 20. 

 The specimen figure 21 is a larger individual, with similar crowded striae and rounded 



cardinal angles. 



In the well-marked specimens of this form, I have not been able to learn the 

 character of the muscular impressions ; and in one small specimen only I have 

 seen a short bifurcating cardinal process. 



Among some obscure specimens from the same localities are some casts of dorsal 

 valves which are much elevated, or obtusely subcarinate along the centre, with 

 duplicating striae. One of these shows a cardinal process and muscular impression, 

 aa illustrated in figure 23 of Plate xi. This may be identical with the preceding 

 form. 



Geological formation and locality. In the Schoharie grit at Clarksville and Knox 

 in Albany county, and at Schoharie. 



Stropliodonta inequiradiata. 



PLATES XI, XII & XIII. 

 Strophodonta inequiradiata : Hail, Tenth Report on the State Cabinet, p. 113. 1857. 



Shell semielliptical or semicircular, more or less gibbous, approaching a 

 hemispheric form ; cardinal extremities sometimes salient, but often 

 rounded, and the cardinal line less than the width of the shell below. 

 The ventral valve varies from moderately convex to gibbous, often 

 forming a somewhat regular arch from beak to front, usually with the 

 greatest convexity above the middle, depressed and often concave. be- 

 tween the umbo and the cardinal extremities, which (in perfect shells) 

 are somewhat salient : beak a little elevated above the hinge-line, and 

 incurved ; area very narrow, linear and finely crenulate. The dorsal 

 valve is concave, often in a less degree than the convexity of the 

 ventral valve ; the disc sometimes very slightly concave, and abruptly 

 deflected or almost geniculate towards the front. 



The surface presents much variety of character and aspect, both in the 

 original shell and in its partial or entire exfoliation. The striae on the 

 ventral valve are often coarse and uneven, somewhat fasciculate, and 



