41 PALAEONTOLOGY OF NEW-YORK. 



Surface marked by fine distinct radiating striaj, those near the cardinal 

 margin 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 ventiicose, and never so distinctly resujiinate. The 

 beaks are more prominent and incurved, that of the dorsal valve curving beyond 

 the hinge-line. The cardinal views of the two species aho differ : 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'^ 

 valves, 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 6, c. Cardinal and dorsal views of the preceding, showing the cavities made by the 

 brachial lamellae 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 oast of a specimen preserving a large and strongly marked muscular impression. 



Fig. 3 6. A mould from the oast shown in the preceding figure. ( The imprints of the ad- 

 ductor muscles are omitted in the drawing.) 



Fig. 8 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 silicification of the shell, and they are not sufficiently curved towards the 

 binge-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 rooks of this age or elsewhere. 



Geological position and locality. In the Oriskany sandstone : Albany and Scho- 

 harie counties ( New-York), Cumljerland ( Maryland ), and other places. 



