36 FIFTEENTH KEPORT ON THE CABINET OF NAT. HISTORY. 



an aperture of about an inch and a half in diameter in both 

 directions; the peristome having a little projection in the postero- 

 lateral margin. The upper side of the spire, for the first one and 

 a half volutions, is flattened ; the angle continuing into the 

 broad expansion of the body -whorl, and dying out before reaching 

 the margin. Aperture nearly round or round-ovate : peristome 

 scarcely sinuous. 

 Surface marked by fine concentric striae, with a few strong spines 

 upon the body-volution. 



This species is conspicuously different from the P. dumosum, in the 

 shorter and comparatively more expanded form ; while the few scattered 

 spines appear to be without order on the surface. A variety, which may 

 belong to this species, has a less expansion of aperture, an obliquely ovoid 

 form, less attenuate apex, and greater proportionate height. 



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

 derberg group : Williams ville, and near Carlisle. The varieties are from 

 Darien, N.Y. 



PLATYCERAS CRASSUM ( n. s.). 



Shell large, irregular, obliquely subovate, arcuate, somewhat 

 broadly flattened on the back, with several more or less strongly 

 defined longitudinal folds, strongly incurved at the summit, the 

 apex making one or two volutions : the body-whorl spreads more 

 on the right side, while the left posterior side is often flattened 

 or depressed, with a greater expansion or sinuosity immediately 

 behind. Aperture very oblique, subquadrangular or irregularly 

 rounded, with a deep sinus on the right anterior margin; the 

 peristome sinuous. 

 Surface marked by concentric undulating lamellose strise : shell 

 thick, raised at unequal intervals into nodes and interrupted 

 ridges. 



The length of large specimens is two and a half inches, and 

 height a little less than two inches; while the transverse diameter 

 of the aperture is from one and a half to two inches, and the lon- 

 gitudinal diameter a little less. 



This shell is remarkable for its great strength and thickness : it is more 

 oblique than the P. rarispinum, and expands less rapidly, while the fold or 

 carina near the summit is on the left side. 



Geological formation and locality. In the Upper Helderberg limestone : 



Hclderberg mountains, Albany and Schoharie counties. 



[ August 



