69 PALEONTOLOGY OF NEW-YORK. 



Gtological formation and locality. This species occurs in the shaly sandstone 

 and Bomicalcareous 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 leoncnsis ( 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 striae, 

 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 thp original shell. 



The species bears some resemblance to the young of O. tioga; but the muscular 

 impressions are diflferent, 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 

 fiize. 



Figs. 8, 4 & 5 of Plate Vili are illustrations of the dorsal valve. 



Fl08. 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. leonensisl is an enlargement of a oast of the ventral valve, where the muscular 



impression in 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 formfltion and locality. In the arenaceous beds of the Chemung 

 group, near Leon, Conewango, and Randolph in Cattaraugus county. 



