GASTEROPODA. 
81 
as above; turning first a little backward and then forward, they make a 
more abrupt retral curve into the umbilical depression. The spaces above 
the upper carina, and between the two carinae and the periphery, are 
distinctly concave, giving prominence to the peripheral band. 
This species was supposed to be only a variety of form of P. trilix , but a 
more critical examination shows it to be quite distinct. The spire is less 
elevated, the last volution more expanded, and the aperture of different form. 
The revolving carinae are much nearer to the peripheral band, which is a 
simple concave space narrower than that above and below it, and limited on 
the two sides by a strong carina. There is no indication of a central line, and 
the curvature of the striae within the band is very different from P. trilix. 
The transverse striae are much stronger, more nearly vertical and distinctly 
bent backward in crossing the carina. It differs conspicuously from P. 
capillaria in the absence of the several cariniform revolving striae above and 
below the periphery, as well as in its stronger concentric striae. 
Formation and localities. In limestone of the Upper Helderberg group, near 
Williamsville, N. Y., and in the same position near Dublin, Ohio. 
Pleurotomaria insolita. 
PLATE XXI, PIG. 17. 
Pleurotomaria insolita, Hall. Illustrations of Devonian Fossils: Gasteropoda, pi. 20, f. 26. 1876. 
Shell turreted ; spire elevated, higher than wide, nearly erect. Volutions 
convex, four or five in the entire shell, gradually enlarging from the apex, 
and the last one moderately ventricose with little lateral expansion ; 
lower side and aperture unknown. 
Surface, above and below the periphery, marked by a sharply defined revolving 
carina, between which and the peripheral band there is a narrow depressed 
space. Above the band the strong transverse striae curve a little back¬ 
ward, scarcely changing their direction or strength in crossing the 
upper carina, and terminating at the peripheral band; below the band 
the striae are nearly vertical to the lower carina, crossing which they 
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