CONTRIBUTIONS TO PALEONTOLOGY. 39 



In this species the shell varies from half an inch to an inch and 

 a quarter; and in a large specimen, the greatest diameter of aper- 

 ture is one inch. 



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

 Moscow and Ludlowville ; and in TuUy limestone? at Ovid, N.Y. 



PLATYCERAS AKGO ( n. s.). 



Shell varying from subdiscoid to obliquely subovoid, with body- 

 whorl extremely ventricose : nucleus minute, with the apex 

 closely enrolled for about two volutions, beyond which it expands 

 more or less abruptly; the last volution nearly or quite in contact 

 with the preceding one. The body of the shell is often obtusely 

 triangular, becoming rounded towards the aperture, and some- 

 times for nearly half the length of the body-whorl. Aperture 

 round or round-ovate, sometimes approaching to quadrangular, 

 broadly sinuate on the right side and deeply sinuate on the left 

 side, where the peristome is sometimes strongly reflexed, forming 

 an apparent columellar lip. 



Structure of the shell lamellose, as in the Cephalopoda, with a 

 nacreous lustre; the exterior surface marked by fine revolving 

 strise, with distant stronger striae or ridges, and cancellated by 

 coarse concentric undulating striae which are bent backwards 

 upon the somewhat regular ridges, presenting several bands simi- 

 lar to the single one in Pleurotomaria. 



This species is remarkable for the peculiar lamellose structure throughout, 

 presented on fracture or exfoliation, and which gives it the character of a 

 Nautilus or Baculites of the Secondary rocks. When the apex remains 

 covered, it might be mistaken for a reversed shell ; the depression on the 

 upper side of the spire being deeper and more abrupt than on the lower side, 

 as the plane of the first volutions is below the centre of the shell, and the 

 spire is shown only in the first or first and second volutions. 



Geological formation and locality. In the Upper Helderberg limestones: 

 Williamsville, Erie county, N.Y. 



GENUS PLATYOSTOMA (Conrad, 1842). 



See Twelfth Annual Report of the Regents on the State Cabinet of Natural 



History, p. 20. 



1861.] 



