]g PALAEONTOLOGY OF NEW YORK. 



volution gradually expanding above and more rapidly towards the aperture, 

 where it has a longitudinal diameter of nearly two and a half inches, the 

 greatest transverse diameter being two inches. The length on the outer 

 curvature is fully four inches; the posterior or concave side is flattened, 

 a rounded ridge being gradually developed and extending to the peristome 

 which is marked by a sinus. A broad obtusely angular ridge extends 

 along the upper side to the base at the right posterior angle. 



A transverse section in the middle of the shell is somewhat pentagonal, 



and the form of the aperture varies from this in having the angles more 



rounded. 



Surface strongly undulated by series of interrupted concentric ridges, which 



doubtless were more fully developed on the surface of the external shell. 



The specimen is essentially a cast, preserving some portions of shell, but 

 not of the external surface. 



The species is sufficiently distinct from any other known in this formation, 

 resembling in some degree the P. subnodosum of the Oriskany sandstone. 



Formation and locality. In limestone of the Upper Helderberg group at 

 Schoharie, N. Y. 



Platyceras crassdm. 



PLATK VII, FIGS. 6-10. 



Platyceras crassum. Hall. Descriptions of New Species of Fossils, etc., p. 8. 1861. 

 " " Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 36. 1862. 



" " " Illustrations of Devonian Fossils: Gasteropoda, pi. 7. 1876. 



Shell large, irregular, obliquely subovate, arcuate, somewhat broadly flattened 

 on the back, with several more or less strongly defined longitudinal folds, 

 strongly incurved at the summit, the apex making one or two volutions ; 

 the body-whorl expanding more on the right side, while the left posterior 

 side is often flattened or depressed, with a greater expansion or sinuosity 

 immediately behind. Aperture very oblique, subquadrangular or irregu- 

 larly rounded, with a deep sinus on the right anterior margin ; the peris- 

 tome sinuous. 



