238 PALEONTOLOGY OF THE EUREKA DISTRICT. 



Surface marked by concentric undulations of growth. 



The original type of this species is the elongate compressed left valve 

 illustrated on plate xix, fig. 5. Subsequently several specimens of the right 

 valve were obtained from the same locality that show more nearly the true 

 outline of the shell. The hinge-line varies in length, as do the relative pro- 

 portions of the height and width of the valves, but these differences do not 

 prevent the union of the several examples under one species. 



In outline some of the shells approach Myalina crtnpla, M. and H. (Name 

 proposed in description of species, Pal. Upper Missouri, p. 33). 



Formation and locality. Lower portion of the Carboniferous Group in 

 the canon northeast of Pinto Peak, Eureka District, Nevada. 



Myaliua Nessus, a. sp. 



Plate xxii, figs. 8, 8a. 



Shell large, subquadrate, narrowing towards the base, higher than wide, 

 compressed posteriorly, becoming somewhat convex on the umbonal slopes. 

 Hinge straight, fully equalling, if not exceeding, the greatest width of the 

 valves, and ranging at nearly right angles to the vertical axis of the shell 

 Beaks terminal and directed obliquely forward. 



Surface marked by concentric lamellaB and undulations of growth. 



Interior characters unknown. 



Dimensions: height of large specimen, 10 cm ; width, about 7 om . 



In size this species approaches Myalina ampla M. and H. (Pal. Upper 

 Missouri, p. 38, 1855), and fully equals M. suJbquadrata Shumard (GeoL 

 Surv. Missouri, vol. 1, pt. 2, p. 207, 1855). It differs from the former species 

 in the outline of the posterior side and in the narrowing of the lower portion 

 of the shell, and also in the direction of the umbonal slope. The same dif- 

 ferences exist between it and M. subquadrata. 



Formation and locality. Lower portion of Lower Carboniferous lime- 

 stone on east slope of a small conical hill on the east side of Secret-canon- 

 road Canon, Eureka District, Nevada. 



