38 FIFTEENTH REPORT ON THE CABINET OF NAT. HISTOEY. 



PLATYCERAS DUMOSTJM, var. RARISPINUM. 



Shell consisting of about two volutions; the apex closely enrolled 

 for more than one volution, and sometimes the volutions are con- 

 tiguous nearly to the aperture : for one and a half volutions the 

 apex is slender and gently expanding, the body-volution expand- 

 ing more rapidly below and becoming moderately ventricose, 

 depressed on the dorsum. Aperture somewhat round-oval. 



Surface marked by wrinkled concentric striae, which are strongly 

 undulated at the bases of the spines. Spines scattered, compara- 

 tively strong, from five to fifteen or seventeen on shells from the 

 medium to the largest size. 

 The larger shells have a length of less than two inches, with a 



height of one and a half inches; the greatest diameter, one inch 



and a quarter. 



This shell is never so ventricose as the P. dumosum proper, and the 

 dorsum is flattened, while in that it is always convex. The P. dumosum is 

 a rare shell, while the smaller variety is not uncommon. 



Geological formation and locality. In limestone of the Upper Helder- 

 berg group : Helderberg mountains, and in Oneida, Onondaga and Erie 

 counties, N.Y.; Canada West ; Ohio, and Falls of the Ohio river. 



The P. dumosum cited by Mr. Conrad as occurring in the Hamilton 

 group, is a variety of more ventricose form than the one here described, and 

 has fewer spines. For this I propose the name Platyceras sjparsum. 



PLATYCERAS ECHINATUM ( n.s.). 

 Shell small, obliquely subovoid. Apex closely incurved for about 

 one volution; the body- whorl, from one-half to one volution, is 

 ventricose, rapidly expanding from the first volution, giving an 

 obliquely conical form. Aperture nearly circular or round-oval : 

 peristome sinuate, the lines of growth and fine striae conforming 

 in direction to the outline of the margin. Remains of revolving 

 striae are sometimes preserved, where the shell is not exfoliated. 

 Besides the concentric and less conspicuous revolving striae, the 

 surface is studded with numerous strong nodes or spines; the 

 latter preserved only when the shell is imbedded in soft shale, 

 and quite separable from the rock : when imbedded in limestone, 

 the spines and exterior are exfoliated with the matrix. 



[ August, 



