GASTEROPODA. (55 



the apex of the shell, and the specimen retaining the shell (fig. 3, plate 17) 

 shows a well defined umbilical depression. One example, broken across the 

 spire, and still preserving the columella, shows an umbilical passage extending 

 above the second volution. 



The specimens are usually much distorted by pressure, and most of them are 

 vertically compressed, so that the entire elevation is less than one inch. 

 Owing to this vertical pressure, the volutions are often angulated at the margin 

 of the periphery. Individuals are occasionally found like figure 4 of plate 17 ; 

 and another still more rotund form is figured on plate 28. It is quite possible 

 that these may prove a distinct species, but the specimens before me offer no 

 means of a satisfactory separation. 



Formation and localities. In the Schoharie grit of the Helderberg mountains, 

 and at Schoharie, N. Y. A few specimens — casts of the interior — from the 

 Upper Helderberg limestone in Western New York, preserve the form and 

 proportions of the compressed specimens from the Schoharie grit of the 

 Helderberg mountains. 



Pleukotomabia akata var. clausa. 



PLATE XVII, FIGS. 9, 10. 

 Pleurotomaria sp. t Hall. Illustrations of Devonian Fossils : Gasteropoda, pi. 17. 1876. 



The specimen figured is a cast of the interior, preserving the general form 

 of P. arata, and is especially similar to that species in the depressed convexity 

 of the upper side of the volutions. The aperture is transverse, broad oval. 

 There is scarcely any evidence of an umbilical opening. Some portions of the 

 shell in a crystalline condition still remain upon the lower side. 



Formation and locality. In limestone of the Upper Helderberg group, at 

 Clarence Hollow, N. Y. 



This form occurs in a fine grained limestone, associated with P. Lucina and 

 P. Hebe, and preserves distinctive features in the form of its volutions and ele- 

 vation of the spire, which more nearly resemble P. arata, to which species I 

 have provisionally referred it. 

 9 



