22 PALEONTOLOGY OF THE EUREKA DISTRICT. 



Genus LEPT^INA Dalman. 

 Leptaena Melita H. & W. 



Leptcena Melita Hall & Whitfield, 1877. Geol. Expl. Fortieth Par., vol. iv, p. 208, 

 pi. i, figs. 13, 14. 



This species is given by its authors as occurring in the Lower Silurian 

 rocks of the Eureka District, the exact horizon being unknown. From the 

 examples in the present collection we learn that it ranges from the limestone 

 beneath the Secret Canon shale on the eastern slope of Prospect Mountain, 

 up through the intervening strata to about 500 feet above the base of the 

 Pogonip Group, at which horizon it is found on the ridge next east of the 

 Hamburg Ridge; also southeast of the Jackson mine, and northeast of 

 Adams Hill, Eureka District, Nevada. 



Genus ORTHIS Dalman. 



Orthls Eurekeiisis, n. sp. 

 Plate ix, figs, ti, 8 a. 



Shell small, transversely suboval, strongly convex. The ventral valve 

 is more convex than the dorsal, and elevated along the middle so as to be- 

 come subcarinate; beak small; area unknown. 



Dorsal valve convex, with a strongly marked median depression that 

 gives it a bilobed character; beak short and apparently incurved over a 

 low area. 



Surface marked by about forty strong, simple, radiating striae that 

 extend up to the beak. 



Dimensions. Transverse diameter, 4 mm ; length of ventral valve, S.b 11 "* ; 

 of dorsal valve, 3 mm ; depth of ventral valve, 1.5 mm ; of dorsal valve, about l mm . 



From the nature of the matrix the beaks of the valves are usually 

 broken away with a portion of the shell on the summit, so that the cast of 

 the hinge-teeth of the ventral valve and the muscular impressions of the 

 dorsal valve are shown, as seen in fig. 8 a, plate ix. 



We know of no similar form from this horizon with which to compare 



