8 
PALAEONTOLOGY OF NEW YORK 
Surface marked by fine closely arranged concentric striae, which are crowded 
in fascicles from irregular growth, and are distinctly undulated in crossing 
the folds of the shell. In parts of the surface, where well preserved, fine 
revolving striae are seen. The aperture is subrhomboidal, and the peris¬ 
tome sinuous. 
This form possesses many characters in common with P. carinatum, but there 
is no development of a carina upon the dorsum, though we may consider that 
this feature is represented in the several low folds which are all in the line of 
the deep sinus which existed in the early growth of the shell, and that the single 
carina is represented by the four or five smaller folds. The posterior margin of 
the peristome is likewise in close proximity to the spire, as in P. carinatum. In 
other respects, it presents some characters in common with P. symmetricum. 
Formation and locality. In a calcareous band in the shales of the Hamilton 
group at Darien, N. Y. 
Platyceras Thetis. 
PLATE III, FIGS. 11-16. 
Platyceras Thetis Hall. Descriptions of New Species of Fossils, etc., p. 4. 1861. 
“ “ “ Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 32. 1862. 
“ “ “ Illustrations of Devonian Fossils : Gasteropoda, pi. 3. 1876. 
Shell obliquely arcuate from the base, with the apex incurved, the nucleus 
making barely more than a single minute volution ; gradually expanding 
from the apex to near the aperture, which is sometimes more abruptly 
spreading. The back of the body-whorl is prominent, and a little flattened 
on the left side ; while the right side, from one-third to one-half the length, 
is sometimes marked by two or three longitudinal folds, and often by more 
numerous, finer plications. Aperture a little oblique, nearly round or 
subquadrangular, with the peristome sinuous. 
Surface marked by fine, closely arranged lamellose striae, which are abruptly 
undulated on all parts of the body of the shell. 
In many, and perhaps nearly all specimens, the body of the shell, along a 
line a little to the left of the dorsum, is marked by an abrupt curvature of the 
