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 rCbust, 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 striae. 



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. 13-2:!. . 



I'lutycera* 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, witli 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. 



