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, 

 pi. 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 oi ? 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. 



Acrotbele ? 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 

 rim 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 to a 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 



