6 PAL^EOXTOLOGF OF NEW YORK. 



longitudinal folds (obscure plications) on one or both sides, which become 

 more strongly developed towards the aperture, and are very conspicuous 

 in old shells ; the right side is more expanded than the left, and in some 

 well preserved specimens is nearly twice as wide. Aperture very oblique, 

 rhomboidal or subtriangular, and the peristome sinuous. 

 Surface marked by fine, closely arranged, undulating striae of growth, which 

 are not lamellose. 



This species is very well marked in its dorsal carina and rapidly expanding 

 body-volution, which spreads always more on the right side. The surface, 

 though distinctly striated, is close, and the shell compact; differing in this 

 respect from some of the other species. Having examined more than a dozen 

 individuals, from the length of less than half an inch to that of an inch and a 

 half, I find that the characters mentioned are preserved in a marked degree 

 in all. In the largest specimens, the aperture is a little more than an inch 

 in its greatest diameter, and nearly equal to the height of the shell. 



Formations and localities. In limestones of the Upper Helderberg group, Hel- 

 derberg mountains ; Williamsville, N. Y., and at Sandusky, Ohio ; in the 

 Hamilton group, at Eighteen-mile creek, Darien, Pavilion, Canandaigua and 

 Seneca lakes. 



Platyceras (Orthonychia) attenuatum.. 



PLATE III, FIGS. 1-6. 



Platyceras attenuatum Hall. Descriptions of New Species of Fossils, etc., p. 2. 1861. 



" " " Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 30. 1862. 



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



Not Platyceras attenuatum, Meek: Proceedings Phila. Acad. Nat. Sci., p. 75. 1871. 

 Not P. dumosum v. attenuatum. Meek: Geol. Rep. Ohio: Palaeontology, I. Expl. of plate 20, fig. 1. 



Shell elongate-ovate or conically subovate with' a slender apex, the nucleus 

 making about one volution or one and a half, below which the body- 

 wborl becomes rather abruptly inflated, and thence gradually expands to 

 the aperture, which is very oblique — the anterior side of the peristome 

 being much more extended. 



