68 FIFTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



by compression. The dorso-ventral diameter of the mouth is three inches ; 

 which, allowing for pressure, may have been originally two and a half inches. 

 After making two-thirds of a volution, the diameter is about one inch and 

 three-fourths. 



There have apparently been no strong surface - markings : impressions 

 of the exterior show striae of growth, which are crowded and somewhat 

 wrinkled on the inner margins of the curve. 



Geological formation and locality. In limestone of the age of the Upper 

 Helderberg group : near Milwaukie, Wisconsin. 



GYROCERAS CYCLOPS ( n. s.). 

 Shell large, gradually tapering from the aperture : one or more 



volutions; section circular; siphuncle dorsal. 

 Surface lamellose striate, and at intervals projected in strong fo- 

 liate expansions, which are plicated towards their periphery. 

 In a specimen making nearly one volution, and measuring on the 

 dorsal curve sixteen inches, the dorso-ventral diameter is about 

 three inches at the mouth, and about one inch at the other ex- 

 tremity. The greatest diameter, from the exterior sides of the 

 curve, is about eight inches. 



A far as can be ascertained, the siphuncle is dorsal. The septa are obscure, 

 and have not been clearly observed within less than an inch of each other. 

 Below the last chamber there are two septa, or a thickened septum, making 

 a thickness of an eighth of an inch ; and at an interval of a little more than 

 an inch, another similar feature, and a third at an inch below the second. 

 The strong lamellose extensions are about an inch asunder where the dia- 

 meter of the shell is two inches, and these make a retral curve upon the 

 back of the shell. 



In a fragment of this species given to me by Prof. Andrews of Marietta, 

 Ohio, the foliate expansions extend an inch beyond the plane of the surface 

 of the shell, are strongly plicated upon the distal half of their width, and 

 closely marked by concentric lamellose striae. The larger specimen described 

 is from the Cabinet of the Albany Institute. 



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

 berg group : Helderberg mountains ; and near Columbus, Ohio. 



G^^ROeE^RAS NAIS ( n. s.). o;|/-^-'b>. 

 Shell consisting of two or more volutions, which are somewhat 

 rapidly expanded from the apex towards the aperture. Volutions 

 subangular in the middle of the sides, rounded on the back, with 



[ September, 



