OKISKANY SANDSTONE. 
463 
Fig. 3 l. Diagram of the interior of the dorsal valve, showing hinge-plates, dental sockets, 
apicial foramen, crural processes and crural plate. In the specimen from which 
this figure is taken, these processes are all encased in crystalline matter, the form 
and direction of the parts alone being visible. The slender process at the base of 
the crural plate is broken off; and since its entire length is unknown, the base 
alone is represented. 
Fig. 3 m. A diagram representing a longitudinal section of the shell, and the internal ap¬ 
paratus of the two valves when connected. 
Geological position and locality. In the Oriskany sandstone : Cumberland, Md. 
Rensselseria intermedia ( n. s.). 
Plate CYIII. Fig. 2 a , b, c. 
Shell ovate, narrowed towards the front : valves subequally convex, 
gibbous in the middle and somewhat abruptly declining towards the 
base and baso-lateral margins, the greatest width a little above the 
middle, the rostral end the broader. Ventral valve more convex than 
the opposite, and more elevated along the centre longitudinally in an 
undefined subangular ridge : beak small and neatly defined, curving 
over the umbo of the opposite valve, its truncated extremity being- 
parallel to the plane of the longitudinal axis; deltidial plates minute. 
Dorsal valve regularly convex, the greatest convexity in the middle. 
Surface marked by fine equal radiating striae, which are more strongly 
delineated on the basal and lateral parts of the shell. Concentric lines 
of growth, at unequal intervals, give the lower half of the shell an 
imbricated appearance. Internal structure as in the R. marylandica. 
This species resembles the preceding in its general aspect, but is more ovate, 
narrowing abruptly towards the front; and the margins of the shell are not inflected 
to any considerable degree, though the tendency to this condition is sometimes 
manifest. 
Fig. 2 a. Dorsal view of a well-preserved specimen. 
Fig. 2 b, c. Profile views of different individuals of the same species. 
Geological position and locality. In the Oriskany sandstone : Cumberland, Md. 
