410 
PALAEONTOLOGY OF NEW-YORK. 
Surface marked by fine distinct radiating striae, those near the cardinal 
jnargin being curved outwards from the beak; concentrically marked 
by obscure lines of growth. 
This species bears a general resemblance to 0, oblata of the Shaly limestone; but 
it attains a larger size, is more ventricose, and never so distinctly resupinate. The 
beaks are more prominent and incurved, that of the dorsal valve curving beyond 
the hinge-line. The cardinal views of the two species also dilfer : the hinge-line of 
-the present species arches upwards on each side of the beaks, while in the other it 
is straight. The area is also higher in the Oriskany species than in that from the 
Shaly limestone. 
In the Oriskany sandstone of New-York, this species usually occurs in the form 
of casts. I have obtained fine specimens of the fossils entire, and of the separate 
yalves, from Cumberland, Maryland. 
The casts which I have referred to this species present some variety in the size 
and form of the muscular impressions; but I am unable to find any constant cha¬ 
racters, by which a distinct species may be established. 
PLATE XCI. 
Fig. 1 a , b. The cast of a small individual. 
Fig. 2 a. Ventral view of a larger individual, with a small muscular impression. 
Fig. 2 b, c. Cardinal and dorsal views of the preceding, showing the cavities made by the 
brachial lamellse and cardinal process, and the impression of the longitudinal 
crest. 
Fig. 3. The ventral side of a cast with narrow and small muscular impressions ( perhaps a 
distinct species). 
Fig. 3 a. The cast of a specimen preserving a large and strongly marked muscular impression. 
Fig. 3 b. A mould from the cast shown in the preceding figure. ( The imprints of the ad¬ 
ductor muscles are omitted in the drawing.) 
Fig. 3 c, d, e. Dorsal, profile and cardinal views of the same specimen. 
PLATE XCV. 
Fig. 1. Dorsal view of a specimen preserving the shell. The striae are partially obliterated 
by the silieification of the shell, and they are not sufficiently curved towards the 
hinge-line in the figure. 
Fig. 2. Profile view of the same specimen. 
Fig. 3 & 4. Dorsal and ventral views of a smaller specimen. 
Fig. 5. Interior of the ventral valve. 
Fig. 6. Interior of the dorsal valve. 
Fig. 7- Cardinal view of the dorsal valve, showing the outline and the elevation of the pro¬ 
cesses. The characters shown in these figures are quite sufficient to distinguish 
this species from any known in the rocks of this age or elsewhere. 
Geological position and locality. In the Oriskany sandstone : Albany and Scho¬ 
harie counties ( New-York), Cumberland ( Maryland ), and other places. 
