2 PALEONTOLOGY OF NEW YORK. 



This species is more robust and rapidly expanding than the P. dentalium, and 

 is more enrolled at the apex; but it does not show the longitudinal sulci and 

 ridges which are characteristic of that species. 



This is the species figured in the Report of the Fourth Geological District, 

 and the typical form for which the generic name Orthonychia was proposed. 

 The apex or nucleus of this and of other species is usually solid, and when 

 the shell is removed, the casts show a rounded obtuse apex, which is sometimes 

 scarcely incurved. 



Formation and locality. In the limestone of the Upper Helderberg group, near 

 Buffalo and Williamsville, N. Y. 



Platycekas (Orthonychia) dentalium. 



PLATE I, FIGS. S-S. 



Platyceras (Orthonychia) dentalium Hall. Descriptions of New Species of Fossils, etc., p. 1. 1861. 

 " " " Fifteenth Rep. N. Y. State Cab. Nat Hist., p. 29. 1862. 



" " " " Illustrations of Devonian Fossils: Gasteropoda, pi. 1. l^Tti. 



Shell slender, elongate, subspiral, making about half of one volution in the 

 length of one inch and a half, somewhat flattened obliquely from the 

 base to near the apex ; section subelliptical, with the diameters about as 

 two to three. The middle of the flattened sides is often a little concave, 

 rounded towards the apex, which is minute and abruptly incurved. 



Surface marked by transverse or concentric striae of growth, and by longi- 

 tudinal sulci, which are conspicuous on the lower part of the shell, and 

 give to the transverse striae a strongly undulated character. Aperture 

 oblique. 



In a specimen of one inch and a half in length, the greatest diameter is less 

 than half an inch. 



This species is much more slender and less distinctly spiral than the P. 

 tortuosum of the Oriskany sandstone, and in the same features differs in a 

 greater degree from any of the species known in the Lower Helderberg group. 



Formation and localities. In the limestone of the Upper Helderberg group, 

 near Williamsville and Buffalo, N. Y. 



