146 PALEONTOLOGY OF THE EUREKA DISTRICT. 
Spirifera (M.) setigera occurs in the Lower Carboniferous of the Hureka 
District, and thus gives a direct local continuation in the Carboniferous of 
the smooth shell, fig. 3, plate iii, from the base of the Devonian. 
Formation and localities —Spirifera (M.) undifera occurs in the Lower 
Devonian of Atrypa Peak in the Eureka District, and also at the same 
horizon on Lone Mountain, 18 miles northwest of Eureka, Nevada. 
Genus CYRTINA Davidson. 
Cyrtina Davidsoniy, n. sp. 
Plate iii, figs. 2, 2 a-e. 
Shell small; general shape more or less triangular or subpyramidal; 
hinge line a little shorter than the greatest length of the shell. 
Ventral valve pyramidal and very much deeper than the dorsal, most 
prominent at the beak, which varies in elevation and is straight or a little 
arched over the area; area large, triangular, flat or slightly arcuate, more 
or less angular on the lateral margins; bent back at varying angles to the 
general surface of the dorsal valve; fissure long, narrow, and apparently 
arched, nearly if not quite, over by the pseudo-deltidium; the median sinus 
is obsolete or else merely a depression between two low, rounded plications. 
Dorsal valve semicircular or depressed convex; beak minute and rising 
a very little above the cardinal line; area linear, very narrow; medium 
fold obsolete or else a low, rounded plication similar to those on either side. 
Surface marked by one or two rounded plications on each side of the 
medium sinus and fold, or these may be almost entirely absent, showing 
as slight undulations on the front margins of the valves. Shell structure 
punctate. 
Dimensions: width of valves of largest example obtained, 8""; height 
of dorsal valve, 6™; depth of ventral valve, 5™™. 
This very pretty species of Cyrtina is unlike any known to me. The 
range of variation in the surface plications is considerable, but I do not 
think it would pass into such forms as C. heteroclita, a variety with few pli- 
cations, or C. Hamiltonensis. 
