2 
PAL2E0NT0L0G-Y OF NEW YORE. 
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. 
Platyceras (Orthonychia) dentalium. 
PLATE I, FIGS. 3-8. 
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. 1876. 
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. 
