GASTEROPODA. 9 



striae, indicating a notch in the margin of the peristome. This line is some- 

 times marked by a narrow prominent band, not unlike that of Pleuroto- 

 maria. 



This species differ from P. attenuatum, in being arcuate from the base, in the 

 gradual attenuation towards the apex, and the more closely incurved nucleus 

 and less abrupt expansion. 



An unusual form (fig. 30, pi. 3) has been referred with doubt to this species. 

 It has the general form and expression of P. Thetis, but is much larger than the 

 prevailing forms, and preserves several protuberances on the surface which 

 have the appearance of spine-bases. The surface preserves the fine revolving 

 striae crossing the concentric ones. 



Formations and localities. In shales of the Hamilton group, at Moscow, York, 

 and Ontario county ; also in limestone of the Upper Helderberg group, Albany 

 county, N. Y. 



Platyceras symmetric™. 



PLATE in, FIGS. 17-28. 



Platyceras symmetricuin, Hall. Descriptions of New Species of Fossils, etc., p. 6. 1861. 

 " Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 34. 1862. 

 " " " Illustrations of Devonian Fossils : Gasteropoda, pi. 3. 1876. 



Shell elongate, subovoid, arcuate, incurved nearly in the same plane; nucleus 

 minute, the spire making about one volution or one and a half, when the 

 body-whorl becomes free and rapidly, or somewhat abruptly, expanded ; 

 spreading about equally on the two sides of the dorsum, which is more 

 prominent and sometimes marked by a ridge. Aperture oblique, sub- 

 quadrate or rhomboidal ; margin of the peristome sinuate, and on the 

 posterior side distant from the spire. 



Surface marked by concentric undulating striae, and longitudinally by obscure 

 interrupted ridges, which, on some parts of the older shells, become regular 

 and uniform, with a narrow groove between. 



This species is well marked by the equilateral expansion on each side of the 

 dorsum, and by the volution of the apex being nearly in the same plane. The 

 2 



