14 PALEONTOLOGY OF NEW YORK. 



is ventricose, rapidly expanding from the first, giving an obliquely 

 conical form. Aperture nearly circular or broad oval ; peristome sinuate, 

 tlu 1 lines of growth and fine striae conforming in direction to the outline 

 of the margin. Remains of revolving striae are sometimes traceable, 

 when the shell is not exfoliated. Besides the concentric and less con- 

 spicuous revolving striae, the surface is studded with numerous nodes or 

 small spines — the latter preserved only when the shell has been imbedded 

 in soft shale, and quite separable from the rock ; when imbedded in lime- 

 stone, the spines and exterior shell are removed with the matrix. 



In this species the shell varies from half an inch to an inch and a quarter in 

 length, and in a large specimen, the greatest diameter of aperture is one inch. 



In this form we have a miniature representation of the P. dumosum. and it 

 might perhaps be considered as the young of that species if occurring in the same 

 formation. Since, however, it is known only in a higher geological horizon, it 

 must be regarded as a distinct species or a degenerate condition of P. dumosum. 



Formation and localities. In shales of the Hamilton group ; Moscow and Lud- 

 lowville ; and in Tully limestone, at Ovid, N. Y. 



Platyceras dcmosum. 



PLATE V, FIGS. 11-16, AND PLATE VI, FIG. 1. 



Platyceras dumosum, Cosrad. Third Ann. Rep. Pal. Dept. [N. Y.] Surv., p. 205. 1840. 



(Cosb.) Hall. Twelfth Rep. N. Y. State Cab. Nat. Hist., p. 19. 1859. 

 " " " Descriptions of New Species of Fossils, etc., p. 9. 1861. 



" " " Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 37. 1862. 



" " " Illustrations of Devonian Fossils: Gasteropoda, plates 5 and 6. 1876. 



Compare Platyceras multispinosum, Mkkk. Proc. Acad. Nat. Sci., Philadelphia, p. 17. 1871. 



" " " " Geolog. Surv. Ohio: Pal., vol. 1, p. 210, pi 20, f. 7. 1873. 



Shell subovoid, arcuate, extremely ventricose when full-grown ; the length 

 from the apex to the anterior margin of the aperture greater than the 

 height. Apex minute, closely enrolled for a single volution or more, 

 when the body-volution becomes free and rapidly expanded, spreading 



