FOSSILS OF THE DEVONIAN. 



183 



what flattened on the left side. Aperture varying from irregularly quadrate 

 to subquadrangular; peristome sinuous and in some examples marked by a 

 prolongation of the dorsal side and a strong sinus on the right posterior 

 margin, which in other specimens is quite shallow and carried back on the 

 posterior margin. 



Surface, as shown by the cast, strongly striate, parallel to the margin 

 of the peristome, and marked by undulations of growth on the body whorl. 



This species is allied to P. Thetis Hall (Pal. N. Y., vol. v, pt 2, p. 8, pi. 

 iii, figs. 11-16) in the general form of the body whorl, but differs widely in 

 its more oblique, arcuate upper portion, a feature distinguishing it from P. 

 symmetricum Hall (loc. cit., p. 9, pi. iii, figs. 17-25) and other allied species. 



Formation and locality. Lower Devonian horizon of Comb's Peak, Eu- 

 reka District, Nevada. 



Platyceras uodosum Conrad. 

 Plate vi, figs. 5, 5 a, 6. 



Platyceras nodosus Conrad, 1841. Fifth Ann. Eep. Pal. State of N. Y., p. 56. 



nodosum Hall, 1859. Pal. N. Y., vol. iii, p. 473, pis. cxv, figs. 1-6 ; cxvi, figs. 1-4. 

 dumosum and P. dumosum, var. rarispinum Hall, 1876. Pal. N. Y., Illustra- 

 tions of Devonian Fossils, Gasteropoda, plates v and vii. 

 nodosum Hall, 1879. Pal. K Y., vol. v, pt. 2, p. 17, pi. vii, figs. 4, 5. 



We have very little hesitancy in identifying the species from Lone 

 Mountain with specimens from the Oriskany sandstone of Knox, N. Y., and 

 the figures of P. nodosum given by Professor Hall. The Nevada shell may 

 expand a little more rapidly along the inner volutions, but of this there is 

 scarcely any proof. The general form and nodose character of the surface 

 vary little from the same features in the New York specimens ; not as much 

 as the specimens from the same locality vary from each other. 



In New York the species ranges from the Oriskany sandstone to the 

 Upper Helderberg Group. In Nevada it is as yet known only from the 

 Lower Devonian in association with other Upper Helderberg species. 



Formation and locality. Lower Devonian of Lone Mountain, 18 miles 

 northwest of Eureka, Nevada. 



