GASTEROPODA. 
71 
Surface marked by fine, even, concentric striae which bend gently backward 
from the suture to near the peripheral band, where they are more abruptly 
deflected, and below which they appear as fine striae of growth. 
This species has the form and proportions of P. sulcomarginata. The 
concentric striae are much finer than those of the prevailing forms of that 
species, and no revolving striae are visible. A careful examination of the 
specimen shows some evidence of the exterior revolving carina, almost obsolete. 
A single specimen only, of the form and characters described, has been 
observed, and, notwithstanding the differences indicated, additional examples 
may show a gradation to some of the varieties of form seen among the large 
collections of P. sulcomarginata from the Hamilton group. 
Formation and locality. In the limestone of the Upper Helderberg group, in 
New York—the particular locality unknown. 
Pleurotomaria rotalia. 
PLATE XIX, FIGS. 20-25. 
Pleurotomaria rotalia, Hall. Descriptions of New Species of Fossils, etc., p. 18. 1861. 
“ “ “ Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 46, pi. 5, tig. 11. 1862. 
“ “ “ Illustrations of Devonian Fossils: Gasteropoda, pi. 19. 1876. 
Shell small, depressed-trochiform; apex minute. Spire consisting of about 
four volutions, which gradually expand to the aperture; slope of the spire 
from the apex to the periphery nearly in the same plane, being very 
slightly convex. Aperture subquadrate; lower side concave, with a com¬ 
paratively large umbilical depression, which is surrounded by a distinct 
areola. 
Surface very finely and closely striate parallel to lines of growth; striae not 
prominent, sometimes obscurely fasciculate, and always bending abruptly 
backward as they approach the narrow peripheral band; obscure indica¬ 
tions of revolving striae are sometimes visible under a magnifier. Suture¬ 
line depressed and narrowly canaliculate, with sometimes a subdued or 
