34 
PALAEONTOLOGY OF NEW-YORK. 
occurrence in the State of New-York. This limestone formation, within 
the limits 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. Species ol tne type of O. eJegamuia and U. subcarinata, with the dorsal valve nearly 
flat : 
Orthis peloris, Orthis lenticularis, 
Orthis 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 muscular 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 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 mu¬ 
scular impression is transversely divided by a ridge, extending on each side from 
the median line, as in species of the type of Orthis elegantula. 
