184 PALEONTOLOGY OF THE EUREKA DISTRICT. 



Platyceras undulatum, n. sp. 

 Plate vi, figs. 2, 2 a. 



Shell small, depressed; apex incurved; body whorl expanding quite 

 rapidly on the outer half of the volution ; dorsum subangular and with a 

 shallow depression on each side; a second rounded carina appears on the 

 left side. The body volution is marked by concentric undulations that are 

 slightly sinuous ; on the dorsal carina a node appears at the crossing of the 

 undulations. Aperture subcircular; peristome sinuate; the undulations 

 corresponding to the outline of its margin. 



Surface of shell unknown. 



There is but one specimen of this species in the collections, but it 

 is so well distinguished by its general form and the presence of trans- 

 verse undulations that there is little danger of confusing it with other spe- 

 cies. It is of the same type as Platyceras Newberryi Hall (Pal. N. Y., vol. 

 iii, p. 33H, pi. Ixiii, figs. 14o-e) of the Lower Helderberg Group, and the 

 undulations in the cast may owe their origin to the presence of rows of 

 nodes or spines on the shell. 



Formation and locality. Lower Devonian, Brush Peak, Eureka Dis- 

 trict, Nevada. 



Platyceras thetiforme, n. sp. 

 Plate vi, figs. 4, 4 a, 6. 



Shell obliquely arcuate from the base with the apex incurved, the nu- 

 cleus making about one volution; body whorl gradually increasing in size 

 to the aperture; on the back it is strongly carinate; the left side is evenly 

 rounded and the right a little flattened, producing a slight carina where 

 uniting with the anterior surface. 



Form of aperture unknown. Surface markings not determined. The 

 strong carina on the back distinguishes this species from P. Thetis Hall, 

 and its gradually expanding body volution, so much like that of P. Thetis, 

 from P. carinatum. 



Formation and locality. Lower Devonian of Brush Peak, Eureka Dis- 

 trict, Nevada, in association with Platyceras Conradi, Platyostoma lineatum, etc. 



