STROPHODONT^ OF THE UPPER HELDERBERG GROUP. 91 



nulated. Dorsal valve moderately concave on the upper and middle part 

 of the shell, a;nd abruptly deflected or geniculate towards the front. 

 Surface marked by fine radiating strise, which are somewhat unequal on 

 the upper and middle portions of the ventral valve, but are fine and 

 regular towards the margin. In some specimens of the dorsal valve, 

 the striae show a tendency to the alternation of three or four finer ones 

 with a distinctly stronger one between ; but in the greater number of 

 specimens, the striae are fine, close, and mostly equal in strength. The 

 radiating striae, when perfect, are crenulated by extremely fine con- 

 centric striae : sometimes a few obscure concentric wrinkles mark the 

 umbonal region, and some specimens show undefined longitudinal folds. 

 In the ventral valve, the occlusor muscular impressions are strongly 

 marked in elongate semielliptical imprints ; while the divaricator muscu- 

 lar impressions are elongate flabelliform, often reaching half the length 

 of the valve. The specimen figured on Plate xiv is shorter than usual. 



In the dorsal valve the muscular impressions are strongly marked, and 

 divided by a low median ridge, which is continued above as a rather 

 strong bifurcate cardinal process : hinge-line rather coarsely crenulated. 



This shell diflfers from S. inequiradiata, in attaining a larger size, and in being 

 more gibbous on the ventral valve. The strise are finer and more equal, and the 

 muscular impressions larger ; while the vascular impressions, so common in casts 

 of that shell, have not been observed in this one. 



Its fine and nearly equal striae, as well as its greater gibbosity, distinguish it 

 from S. patersoni. It approaches very nearly the S. concava of the Hamilton group; 

 and the two have so many characters in common, that I have hesitated to continue 

 them as distinct species. A larger collection of the limestone specimens are 

 needed, before a satisfactory comparison can be made. 



The figure 2 a on Plate Xiv illustrates the exterior of a dorsal valve, where the cardinal 

 extremities are anriculate. The profile view, figure 2 6, is a little less gibbous than 

 usual. 



The interior of the dorsal valve is shown in figure 2c; and the cardinal view, figure 2 d, 

 shows the muscular impressions of the ventral valve. 



Geological formation and locality. This species occurs in the Schoharie grit in 

 Albany and Schoharie counties, and not rarely in the Cornifereus limestone in 

 nearly all localities of this formation throughout the State ; and I have a specimen 



