14 PALEONTOLOGY OF THE EUREKA DISTRICT. 
Formation and localities—Cambrian, Prospect Mountain limestone, 
just beneath the Secret Canon shale, in the 700-foot level of the Richmond 
mine, Ruby Hill; at the summit of the Secret Canon shale in New York 
Canon; also in the lowest portion of the Pogonip limestone northeast of 
Adams Hill and higher up in the same group on the eastern slope of the 
ridge east of the Hamburg Ridge, Eureka District, Nevada. 
Genus OBOLELLA Billings. 
Obolella discoidea H. & W. 
Obolella discoidea, Hall & Whitfield, 1877. Geol. Expl. Fortieth Parallel, vol. iv, p. 205, 
pl. i, figs. 1, 2. 
This is one of the species brought in by the geologists of the Fortieth 
Parallel Survey from the Eureka District. It occurs in the Hamburg 
shale of the Cambrian, and the Pogonip Group of the Silurian, on the 
ridge east of the Hamburg Ridge and at the Pogonip horizon in the lime- 
stone northeast or Adams Hill; also on the first ridge southeast of the Jack- 
son mine, Eureka District, Nevada; and in the Pogonip limestone on Pogo- 
nip Ridge, White Pine District, Nevada. 
Genus ACROTHELE Linnarsson. 
Acrothele? dichotoma, n. sp. 
Plate ix, fig. 11. 
Shell of medium size, thin, corneous, nearly circular in outline, the 
transverse diameter being a little greater than the longitudinal. 
Ventral valve depressed, discoid, with the apex but slightly elevated 
above the general plane of the surface and situated about one-third the 
distance from the posterior to the anterior margin. ‘The apex is perforate | 
and has a narrow groove around it, except on the posterior side, where a low 
vim alone separates the groove and the slightly-depressed pseudo:deltidial 
area, ‘This area is triangular in form, and extends to the posterior margin, 
gradually expanding toa width a little less than its length; it appears to 
have been quite minute at first just beneath the apex and to have been filled 
