g 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 Hali,. 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, pL 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 



