30 PALJEOSTOLOQY OF A/11 YORK. 



Platyostoma pleurotoma. 



II Ml. 1\. HUS. 81-36. 

 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. 



Strophostylux uniea, 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, concentric 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. 



