30 
PALEONTOLOGY OF NEW YORK. 
Platyostoma pleurotoma. 
PLATE IX, PIGS. 31-35. 
Platyostoma pleurotoma, Hall. Illustrations of Devonian Fossils : Gasteropoda, pi. 10. 1876. 
Shell rotund, subturbinate. Spire depressed; volutions rounded, gradually 
enlarging, and the last one much expanded. Aperture broadly oval, and 
extended below; peristome more or less sinuous, and, on the columellar 
side, extended below in a thickened and slightly contorted callosity. 
Surface finely striated with concentric and revolving striae, the latter, con¬ 
spicuous, and the former scarcely perceptible. In one specimen, a 
distinct narrow band marks the suture-line; and in another, a narrow 
carina marks the periphery, giving the aspect of Pleurotomaria. 
Formation and locality. In limestone of the Upper Helderberg group, Onon¬ 
daga, N. Y. 
Strophostylus unicus. 
PLATE XI, FIGS. 14, 15. 
Strophostylus unica, Hall. Descriptions of New Species of Fossils, etc., p. 13. 1861. 
“ “ “ Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 41. 1862. 
“ unicus Illustrations of Devonian Fossils: Gasteropoda, pi. 11. 1876. 
Shell suborbicular ; height and length about as three to four. Spire consisting 
of three or four volutions; the apex moderately elevated above the outer 
volution; suture close, and the volutions evenly convex upon the upper 
side; body-whorl ventricose, evenly and equally rounded upon the back. 
Aperture subovate, a little extended at the lower side, and slightly 
straightened on the posterior side ; outer lip regularly curved; columellar 
lip thickened and slightly twisted, the fold being near the upper part of 
the lip. 
Surface marked by fine, crowded, concentrio- striae, which are broadly curved 
backward on the dorsum; shell very thick. At one point, where the 
shell is broken away, the surface within the cavity is marked by revolving 
striae. 
