32 SIFXEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



dividnals, the muscular impressions present some differences in 

 form, and in the depth of the impressions of the parts, probably 

 indicating the characters of dorsal and ventral valves. 



This species closely resembles one in the Oriskany sandstone, from which 

 it differs in being more ovate, as well as in the form and divisions of the 

 muscular scar, and more depressed posterior side. 



Geological formation and locality. In Schoharie grit : at Clarksville 

 and Knox, Albany county. 



GENUS ORTHIS ( Dalman). 

 " ORTHIS PELORIS ( n. s.)." 

 Shell plano-convex ; the dorsal valve nearly flat, and subcircular 

 or slightly transverse : ventral valve somewhat elliptical, very 

 convex on the umbo, the middle regularly convex and sloping 

 to the front and sides ; cardinal line equal to or greater than 

 two-thirds the greatest width of the shell. Area of moderate 

 height, the beak of the ventral valve extending little beyond 

 the area line. 

 Surface marked by fine somewhat evenly bifurcatiug striae, which 

 curve towards the margins, and a few terminate on the area line 

 of the ventral valve. 



The specimens are casts of the interior, with impressions of the exterior : 

 from these latter, casts have been taken, which have served for the descrip- 

 tion of the exterior surface of the shell. 



The internal cast of the ventral valve is highly convex ; the greatest 

 convexity being above the centre, sloping gently to the front and more 

 abruptly to the sides. The muscular impression is comparatively small, sub- 

 elliptical in form, the longitudinal being a little greater than the transverse 

 diameter ; deeply bilobed by the callosity of the adductor muscle, and the 

 sides somewhat distinctly lobed in about three divisions. The surface of the 

 cast, below the muscular impression, is faintly marked by the vascular 

 impression, and the margin strongly striated. The distance from the beak, 

 or filling of the rostral cavity, to the lower side of the muscular impression, 

 is less than half the length of the cast. 



The cast of the dorsal valve is nearly flat, with an oval muscular area 

 which is deeply divided in the centre from the pit made by the cardinal 

 process and its extension along the interior of the shell, whence it becomes 

 bifurcate. The muscular impression is transversely divided by a ridge ex- 

 tending from the median line, as in specien of the type of Orthis clegantula. 



The casts of these two valves, which I have placed under one designation, 

 have not been found in actual connexion ; but the character of the ventral 

 valve requires a dorsal valve of the form described, and vice versa; while 

 the two occurring in the same locality, and of equal rarity, I can feel little 

 hesitation in considering them of the same species. 



Geological formation and locality. In the Schoharie grit : at Clarksville 

 and Knox, Albany county. . 



