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 ampla, 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. 
Myalina Nessus, n. 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 lamellze and undulations of growth. 
Interior characters unknown. 
Dimensions: height of large specimen, 10°; width, about 7™. 
In size this species approaches Myalina ampla M. and H. (Pal. Upper 
Missouri, p. 33, 1855), and“fully equals Jf. subquadrata Shumard (Geol. 
Sury. 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 JZ. 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. 
