SI PALEONTOLOGY OF NEW-YORK. 



occurrence in the Stute of New- York. This limestone formation, within 

 the limit-s of the State, being for the greater extent a firm and subcrystal- 

 line rock, has yielded few specimens of as great perfection as those of the 

 Lower Helderberg group. 



A. S|)fcie8 ot tne lypo of O. tlegantula and V. Btibcarinata, witli the dorsal valve Jiearly 



flat : 

 Orthis fcloeis, Ortiiis lenticularis. 



Ortliis peloris ( n. s.). 



PLATE IV. 

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

 slightly transverse : ventral valve somewhat semielliptical, very con- 

 vex 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 bifurcating 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 description 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 ranscnlar impression is comparatively small, subelliptical in form, the lon- 

 gitudinal being a little greater than the transverse diameter, deeply bilobed by 

 the callosity of the adductor muscle, and the sides somewhat distinctly lobed iu 

 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 tlie 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 mu- 

 scular impression is transversely divided by a ridge, extending on each side from 

 the median line, as in species of the typo of Orthis cleganlula. 



