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 striae, 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 strife extend, with 
10 
