OBISKANY SANDSTONE. 
413 
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 Productus (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 becoming 
filled with the material forming the cast. The principal differences described are 
shown in the figures of the dorsal valves of the two species. 
The figures 2 b and 2 c, Plate XCIII, are referred to this species with some hesitation : 
they are 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. 
