si FIFTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



PLATYCEKAS THETIS ( n. s.). 

 Shell obliquely arcuate from the base, with the apex incurved, and 

 making scarcely a single minute volution; gradually expanding 

 from the apex to near the aperture, which is sometimes more 

 abruptly spreading. The back of the body-whorl is prominent, 

 and a little flattened on the left side; while the right side, from 

 one-third to one-half the length, is sometimes marked by two or 

 three longitudinal folds. Aperture a little oblique, nearly round 

 or approaching to quadrangular, with the peristome sinuous. 

 Surface marked by fine closely arranged lamellose striae, which 

 are abruptly undulated on all parts of the body of the shell. 

 In many, and perhaps nearly all specimens, the body of the shell, 

 along a line a little to the left of the dorsum, is marked by an ab- 

 rupt curvature of the striae, indicating a notch in the margin of the 

 peristome. This line is sometimes marked by a narrow prominent 

 band, not unlike the band in Pleurotomaria. 



This species differs from P. attenuatum, in being arcuate from the base, 

 in the gradual attenuation towards the apex, and the more closely incurved 

 nucleus. 



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

 at Moscow, York, and Ontario county ; and in limestone of the Upper Hel- 

 derberg group, Albany county, N.Y. 



PLATYCERAS ERECTUM. 



AcROcuLiA ERECTA : Geol. Report 4th District New-Tork, p. 174, and flg.G.. p. 172. 

 This species, originally described from specimens in the Corniferous lime- 

 stone, occurs also in the Hamilton group. The spire at the apex is closely 

 enrolled for about one and a half volutions, beyond which the body-volution 

 becomes somewhat rapidly expanded, with the aperture often spreading. 

 The specimens are often more arcuate than the figure in the Geological 

 Report, and the aperture oblique, with the peristome sinuate. The surface 

 is marked by closely arranged revolving lamellose striae, which, upon the 

 lower half of the body- volution, are abruptly arched along narrow bands 

 corresponding with former sinuosities of the aperture. 



Geological formation and locality. In the limestone of the Upper 



Helderberg group, Williams ville and near Buffalo ; and in the Hamilton 



group, at York, Moscow, Darien, and other places. 



[ August, 



