146 PALEONTOLOGY OF THE EUREKA DISTRICT. 



Spirifera (If.) setigera occurs in the Lower Carboniferous of the Eureka 

 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 (Jlf.) 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 Davidson!, 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 is obsolete or else merely a depression between two low, rounded plications.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 mm ; height 

 of dorsal valve, 6 mm ; depth of -ventral valve, 5 mm . 



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. heterodita, a variety with few pli- 

 cations, or G. Hamiltonensis. 



