10 PALAEONTOLOGY OF NEW YORK. 



posterior margin of the aperture is widely separated from the preceding volu- 

 tion. The longitudinal ridges are more strongly marked, and of a different 

 character from those of characteristic specimens of P. bucculentum; it is not, 

 however, improbable that we may find intermediate forms uniting the two 

 species. It is also possible that a comparison of a larger number of speci- 

 mens may prove that the forms included under P. Thetis, P. symmetricum and P. 

 bucculentum are varieties of one species. 



Formations and localities. In the Upper Helderberg limestone at Darien, and 

 in the Hamilton group at Darien, York, and Canandaigua lake, N. Y. 



Platyceras bucculentum. 



PLATE in, FIGS. 7, 26-29. 



Platyceras bucculentum, Hall. Descriptions of New Species of Fossils, etc., p. 5. 1861. 

 » " " Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 33. 1862. 



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



Shell ventricose, obliquely subovoid. Apex extremely attenuate, the spire 

 making one or two closely enrolled volutions, with a gently enlarging 

 diameter, and below this abruptly expanding and becoming very ventricose 

 in the middle and lower part ; spreading more upon the right side than 

 upon the left ; the shell near the posterior side swells out into a distinct 

 pouch-like projection, with two or three rounded folds or semiplications, 

 which give a deeply sinuous outline to the margin. Aperture subovate, 

 and sinuate on the right posterior side. Peristome sinuous, and on the 

 posterior side spreading partially over the preceding volution. 



Surface marked by fine closely arranged concentric stria?, which are undulated 

 towards the margin of the aperture, and sometimes over the greater part 

 of the surface, the irregularity having commenced during the earlier stages 

 of growth. In well-preserved specimens there are revolving striae or 

 fascicles, rising in little bands of obsolescent striae, giving a waved aspect 

 to the surface. 



This species is more ventricose than any of the preceding forms. The char- 

 acter of apex, and the widely expanded body- volution resemble in some degree 



