30 FIFTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



This is the species figured in the Report of the Fourth Geological District, 

 and the form typical of those for which I proposed the generic name Or- 

 THONYCHIA. The apex or nucleus of this and of other species is usually 

 solid, and, when the shell is removed, the casts show a rounded obtuse apex 

 which is sometimes scarcely incurved. 



Geological formation and locality. In the limestone of the Upper 

 Helderberg group : near BuiFalo and Williamsville, N.Y. 



PLATYCERAS ATTENUATUM ( n. s.) 



Shell elongato-ovate or conically subovate, with a slender apex 

 which makes about a single volution, and below which the body- 

 whorl 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. 

 Surface marked by crowded undulating concentric strise and lon- 

 gitudinal irregular and undefined folds, which vary greatly in 

 different specimens : these become more distinctly marked as 

 plications near the aperture. Peristome sinuous, with numerous 

 indentations corresponding to the folds upon the surface. 

 In many specimens the surface is marked by abruptly undulating 

 plications without distinct folds, or with the folds obscurely deve- 

 loped. Length of shell about one inch, with the greatest diameter 

 a little less than half an inch. 



This species is distinguished by the abrupt contraction of the upper part 

 of the shell of the body-whorl, or just below the curvature ; while the apex, 

 consisting of a single minute volution, is abruptly contracted, and propor- 

 tionally more slender than in most other species. It somewhat resembles the 

 P. clavatum of the Lower Helderberg group. 



Geological formation and locality. In the shales of the Hamilton group 

 on the shores of Seneca and Cayuga lakes, Greneseo, Moscow, and other 

 places in Western New- York. 



PLATYCERAS (ORTHONYCHIA) CONCAVUM (n.s.). 

 Shell robust, subspiral, slender, gradually expanding above and 

 more rapidly dilating towards the aperture, which is subquadri- 

 lateral, with the peristome strongly undulated. 



[ August, 



