GASTEROPODA. 
3 
Platyceras (Orthonychia) concavum. 
PLATE I, FIGS. 9-12. 
Platyceras ( Orthonychia ) concavum Hall. Descriptions of New Species of Fossils, etc., p. 2. 1861. 
“ “ “ “ Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 30. 1862. 
“ “ “ “ Illustrations of Devonian Fossils: Gasteropoda, pi. 1. 1876. 
Shell robust, subspiral, slender, gradually expanding above and more rapidly 
dilating towards the aperture, which is subquadrilateral, with the peristome 
strongly undulated. The specimen is imperfect at the apex, and the lower 
portion makes less than half a turn from the aperture in the length of 
about two inches; apex unknown. Anterior side of the body-volution 
convex and ventricose; posterior side distinctly concave ; the right side 
moderately convex, and the left side rounded. 
Surface marked by concentric undulating strife. 
The length of the largest fragment is about two inches; the aperture from 
the posterior to the anterior side is about seven-eighths of an inch, and the 
transverse diameter a little more than one inch. 
This species resembles the P. tortuosum of the Oriskany sandstone, but is 
more robust and more rapidly expanding towards the aperture, which is much 
less oblique than in that species, while the concave posterior, and prominent 
anterior sides are distinguishing features. A comparison with a larger number 
of specimens of P. dentalium suggests that this form, in figs. 9 and 10, may be 
only a larger variety of that species. 
Formation and locality. In the limestone of the Upper Helderberg group, 
Williamsville, Erie county, N. Y. 
Platyceras (Orthonychia) conicum. 
PLATE I, FIGS. 13-23. 
Platyceras conicum Hall. Descriptions of New Species of Fossils, etc., p. 3. 1861. 
“ “ “ Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 31. 1862. 
“ “ “ Illustrations of Devonian Fossils : Gasteropoda, plates 1 and 2. 1876. 
Shell erect, conical, the minute apex closely incurved. Body-volution entirely 
straight, with broad undefined longitudinal ridges and depressions, which 
become more distinct towards the aperture; height of the shell a little 
greater than the width of aperture, which is a little longer than wide. 
