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 symmetricum. 
PLATE III, FIGS. 17-25. 
Platyceras symmetricum, 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 
