62 
PALAEONTOLOGY OP NEW-YORK. 
Geological formation and locality. This species occurs in the shaly sandstone 
and semicalcareous bands of the Chemung group, at Ithaca and Cayuta creek; at 
Chemung-narrows, and near Elmira ; also at Philipsburgh in Allegany county, and 
at Leon and elsewhere in Cattaraugus county. 
Ortliis leonensis (n. s.). 
PLATE VIII. 
Shell transverse, somewhat broadly elliptical. Dorsal valve with a dis¬ 
tinct longitudinal sinus, and having the base more or less distinctly 
truncate in the middle; the sides, for a little way below the beak, are 
sometimes straight, but usually curved. Ventral valve regularly convex 
in the middle and upper part, sometimes a little depressed towards the 
base and elevated along the median line : muscular impression oval 
or ovate, and sometimes visibly lobed. The area is much shorter than 
the width of the shell. The surface is marked by rather strong strise, 
which are more or less preserved upon the cast. 
The strength of the muscular markings, as well as of the striae, vary 
considerably, depending on the thickness or strength of the original shell. 
The species bears some resemblance to the young of 0. tioga; but the muscular 
impressions are different, and the sinus in'the dorsal valve is quite unlike that one. 
A considerable number of specimens have been found, but among them there are 
no large individuals, and it would appear that the species never attained a large 
size. 
Figs. B, 4 & 5 of Plate vill are illustrations of the dorsal valve. 
Figs. 6 & 7 are the ventral valves. 
Fig. 8 represents the surface of a larger ventral valve which may possibly be of this species, 
as it occurs in the same association. 
Fig. 1, O. leonensis? is an enlargement of a cast of the ventral valve, where the muscular 
impression is much stronger than usual, and the surface below is much flattened ; a 
feature which may possibly have come from great thickening of the shell, or it may 
be a different species. 
Geological formation and locality. In the arenaceous beds of the Chemung 
group, near Leon, Conewango, and Randolph in Cattaraugus county. 
