ORISKANY SANDSTONE. 4f3 



broader ones the depressed portions ( in this respect the reverse of the 

 preceding species). Surface of the cast striato-punctate. 

 The dorsal valve (which I have inferred may belong to the same species) 

 is gently concave in the middle, and more abruptly bent outwards to- 

 wards the margin. The double cardinal process is strongly developed. 

 A cast from the mould in sandstone shows a large area, marked by the 

 internal vessels. The areas for the attachment of the cardinal and ad- 

 ductor muscles are narrow and elongated ; while below, and directed 

 outwards, is a curving line, marking a space similar to the vascular 

 impressions of the dorsal valve of Produdus ( these lines are not pro- 

 perly shown in the figure). The ovarian spaces are strongly corrugated, 

 and the lower part of the area is marked by elevated ramifying and 

 diverging lines, while the margin outside of this limit is simply striato- 

 punctate. 



This species, in the dorsal valve, differs from the preceding in the narrower 

 muscular impressions, and a more strongly marked curving line from the base of 

 the cardinal muscles, while the space it encloses is less prominent : the ramifying 

 muscular impressions in the lower part of the area are more prominent and more 

 divergent. I have not seen the valves in connexion, nor any other portions of the 

 fossil except the impressions left in the sandstone. 



The differences indicated in the casts seem to me sufficient to warrant the separa- 

 tion of these forms; the latter one, indeed, showing in its dorsal valve more affinity 

 with the following than with the preceding species. 



PLATE XCII. 



Fig. 4. A cast of the ventral valve. ( The muscular and vascular markings are not correctly 

 represented in the figure.) 



PLATE XCV. 

 Fig. 10. A cast of the dorsal valve from an impression in sandstone. The divisions of the 

 cardinal process are not of the full length, owing to the cavities not hecoming 

 filled with the material forming the cast. The principal difi'erences described are 

 shown in the figures of the dorsal valves of the two species. 



The figures 2 h and 2 c, Plate XCIII, are referred to this species with some hesitation : 

 they arc the impressions of the exterior of the dorsal valve, and show the convexity corre- 

 sponding to the concavity of the shell, which only occur in an equal degree in this or the 

 preceding species ; the others known in the rock being much less concave on the dorsal valve. 



Geological position and locality. In the Oriskany sandstone : Helderberg moun- 

 tains, Albany county. 



