74 PALEONTOLOGY OF THE EUREKA DISTRICT. 



Orthis Lonensis, n. sp. 

 Plate xi, figs. 6, Ca. 



Shell of medium size, transversely semi-oval, width and length as 4 to 

 3; hinge line about equal to the greatest breadth of the valves. 



Ventral valve elevated, convex; the greatest convexity a little behind 

 the middle on each side of a shallow, broad mesial sinus; beak small, not 

 projecting backward; area high at the beak, sloping rapidly to the lateral 

 margins; slightly arched, and standing at nearly right angles to the plane 

 of the valve; foramen a little higher than broad, with its apex extending to 

 the termination of the beak. Dorsal valve unknown. 



Surface ornamented by distinct radiating striae that increase by inter- 

 calation, and curve gradually outward to the postero-lateral margins. 



This species belongs to that group of Orthis, of which 0. occidentalis is 

 a type, and appears to be specifically distinct from any described species. 



Formation and locality. Pogonip Group, upper beds, Lone Mountain, 

 18 miles northwest of Eureka, Nevada. 



Orthis tricenaria, Coiirad. 

 Plate xi, figs. 4, 4 a. 



Orthis tricenaria Conrad, 1843. Proc. Acad. Nat. Sci., vol. i, p. 333. 



Orthis tricenaria Hall, 1852. Pal. N. Y., vol. i, p. 121, pi. xxxi, figs. 8 a, c. Ibid., Geol. 



Surv. Wisconsin, vol. i, p. 42, figs. 8-11, 1862. 

 Orthis tricenaria Billings, 1863. Geol. Canada, p. 167, figs. 151 , &. 



An examination of a good series of specimens of this species shows that 

 it varies a little in the number and strength of the surface striae in both the 

 young and old shells, the variation extending from the average number 

 of 28 to 30. to 16 as the minimum and 35 as the maximum; with this in 

 view, the specimens from the Pogonip Group (upper beds, at Lone Mount- 

 ain and Surprise Peak) cannot well be specifically separated from those of 

 Wisconsin and New York. The general form of the shell is the same, and 

 the number of radii varies from 16 to 30. Some of the examples are almost 

 typical forms of Orthis costalis, of the Chazy limestone of New York, and a 

 comparison of specimens shows the two species to be closely allied. It may 

 be considered that the Nevada form is a variety of 0. tricenaria, and worthy 



