34 FIFTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



more upon the right side than upon the left, and the peristome 

 near the posterior side swelling out into a distinct pouch-like 

 projection, with two or three rounded folds or semiplications 

 which give a deeply sinuous outline to the margin. Aperture 

 subovate, and sinuate on the right posterior side. Peristome 

 slightly sinuous, and spreading on the posterior side partially 

 over the preceding volution. 

 Surface marked by fine closely arranged concentric strise, undu 

 lated towards the margin of the aperture; with obsolete revolving 

 bands or lines, giving a waved aspect to the surface. 



This species is more ventricose than any of those described in this paper. 

 The character of apex, and the widely expanded body-volution, resemble 

 in some degree the P. ventricosum of the Lower Helderberg ; but the iirst 

 volution is more slender and the spire less elevated, while the lateral pouch- 

 like expansion is a distinctive feature. This shell is also much smaller, rarely 

 more than an inch high, while the greatest diameter of the aperture is about 

 equal to the height. 



Geological formation and locality. In shales of the Hamilton group at 

 York and Moscow, Livingston county, N.Y. 



PLATYCERAS SYMMETRICUM ( n. s.). 



Shell elongate, subovoid, arcuate, incurved nearly in the same 

 plane, the apex making about a single volution, below which the 

 shell is somewhat abruptly expanded : expansion about equal on 

 the two sides of the dorsum, which is more prominent and some- 

 times marked by a ridge. Aperture very oblique : margin of the 

 peristome sinuate, and, on the posterior side, distant from the 

 spire. 



Surface marked by concentric undulating strise, and longitudinally 

 by obscure interrupted ridges, which, on some parts of the older 

 shells, become regular and uniform with a narrow groove between. 



This species is well marked by the equilateral expansion on each side of 

 the dorsum, and by the volution of the apex being nearly in the same plane. 

 The longitudinal ridges are more strongly marked, and of a different cha- 

 racter from those of P. bucculentum. 



Geological formation and locality. In the Upper Helderberg limestone 

 at Darien, and in the Hamilton group at Darien, York, and Canandaigua 

 lake. 



[ August, 



