FOSSILS OF THE LOWER SILURIAN. 



i i 



The surface presents a smooth, glistening appearance, and is marked 

 by fine concentric striae and very fine, often scarcely perceptible radiating 

 striae. This surface gives to, fragments of the shell the character of some 

 of the Linguloid shells. 



The species belongs, by its general form, to the group of which T. 

 (Ctenodonta) varicosa Salter (Mem. Geol Surv. Gt. Brit, vol. iii, p. 345) and 

 T. (C ) astartceformis Salter (Can. Org. Remains, dec. 1, p. 39, pi. viii, fig. 7) 

 are the types; specifically it is distinct. 



Formation and locality. Upper part of Pogonip Group. East slope of 

 ridge east of the Hamburg Ridge, Eureka District, Nevada. 



Genus MODIOLOPSIS Hall. 



Modiolopsis occidens, n. sp. 

 Plate i, fig. 5; pi. xi, figs. 14, 14 a. 



Shell small, elongate, ovate in outline; broadest at the posterior half, 

 contracting to about two-thirds the greatest width at the beaks; hinge-line 

 nearly straight and a little more than half as long as the shell posterior to 

 the beaks; posterior margin slightly convex and sloping obliquely from the 

 extremity of the hinge-line to the rounded posterior end of the shell; the 

 basal line curves downward, and then with a slight sinuosity, opposite the 

 beak, extends nearly straight to the somewhat narrow, sharply rounded, 

 anterior end; beak small, scarcely extending above the hinge-line, and sit- 

 uated within the anterior third of the shell. The surface is moderately 

 convex, most prominent along the posterior umbonal ridge, whioh is rather 

 broadly rounded; umbonal slope abrupt and slightly convex A broad, 

 obscure mesial depression is shown on the lower part of the anterior half of 

 the shell that does not extend up to the beak. 



Anterior muscular scar of medium size and well defined; posterior 

 scar, pallial line, and exterior surface markings unknown. 



This species is related to Modiolopsis mytiloides Hall (Pal. N. Y., vol. 

 i, p. 157, pi. xxxv, figs. 4 a, b] in its general outline, but differs in its less 

 elongate form and in its position of its beaks. 



Formation and localities. Pogonip Group, upper portion, Fish Creek 



