GASTEROPODA. 73 



The revolving striae below the periphery are somewhat irregular and less 

 conspicuous than on the upper side of the volutions, and the concentric 

 striae are much subdued. 



This, so far as known, is a rare species. It may be distinguished from P. 

 sulcomarginata by the numerous strong revolving striae upon the upper side of 

 the volutions, and the fine cancellating striae : the apex is less acute, and the 

 volutions more regularly expanding. The columella is not extended and 

 thickened as in that species, and the lower side of the volution preserves the 

 two sets of striae. In the character of surface-striae it resembles P. Hebe, but 

 the spire is less oblique, the upper margin of the periphery is not carinate, and 

 the body of the shell is less ventricose below, as may be seen by comparison 

 with figures 5 and 6 of plate 19. 



The species bears considerable resemblance to P. radula of de Koninck, but it 

 seems quite distinct from any other described American species. 



Formation and locality. In the shales of the Hamilton group, at York, Liv- 

 ingston county, N. Y. 



Pleurotomaria filitexta. 



PLATE XX, FIGS. 26, 27, and PLATE XXVIII. 

 Pleurotomaria filitexta, Hall. Illustrations of Devonian Fossils : Gasteropoda, pi. 19. 1876. 



Shell trochiform, substance thin ; spire depressed-conical, height about one- 

 sixth less than the width. Volutions four, five or more; very depressed- 

 convex, scarcely rising above the plane of the spire, gradually enlarging 

 above, the last one or two more rapidly expanding, and becoming very 

 ventricose towards the aperture, which is somewhat rhomboidal. Periphery 

 obtusely angular ; lower side very convex, and abruptly depressed towards 

 the umbilical region ; columella extended. 



Surface marked by fine, regular, concentric and revolving stria?, which are 

 nearly equal in strength on the upper part of the shell, while the concen- 

 tric striae are more conspicuous on the lower volutions, and especially 

 towards the aperture. From the suture the concentric striae extend, with 

 10 



