FOSSILS OF THE LOWER SILURIAN. 233 



variable in proportion; beak obtuse, and rather prominent; the cardinal 

 margins sloping rapidly to the extremities of the hinge. Area not observed, 

 but, judging from the prominent beak, it must be rather high and but slightly 

 arcuate; hinge-extremities angular, or slightly rounded. Surface of the 

 valves strongly convex and marked by extremely fine striae, which become 

 a little coarser toward the margins of the shell. Intenor unknown. 



The species has much the general form of 0. costaUs Hall of the New 

 York Chazy limestone, but the strise differ greatly in their extreme fineness. 

 It also somewhat resembles 0. Electra Billings (Pal. Foss. ' Canada, vol. 1, 

 p. '(9, fig. 72), but is not so broad, and the valves are more convex, and 

 beak more prominent. The shells occur in a coarsely granular and highly 

 crystalline limestone, and become more or less exfoliated in separating from 

 the matrix, so that the external features are not perfectly recognized. The 

 examples used are all separated valves, partially imbedded in the rock, so 

 that the hinge-features are not fully revealed. Those figured would seem 

 to be dorsal and ventral, and, if so, the dorsal is much less convex than the 

 ventral. 



Formailon and locality — ^In limestone of the age of the Chazy of New 

 York, Pogonip Mountain, White Pine District, Nevada. Collected by A. 

 J. Brown, esq. 



Genus STROPHOMENA Rafinesque. 

 Strophomena Nemea n. sp. 



Plate I, fig. 15. 



Among the sjiecimens received from the gray granular limestones of 

 the Pogonip Mountain, there is a small Strophomenoid shell, which is some- 

 what semi-oval in outline, about two-thirds as high as wide; the hinge-line 

 nearly straight and not quite as long as the width of the shell below; the 

 cardinal angles are slightly rounded; and the lateral margins and basal line 

 almost regularly curved, forming a little more than half of a circle. The 

 surface of the ventral (?) valve is regularly and evenly convex longitudi- 

 nally and laterally, but not highly rounded, and marked by rather fine, 

 even, and somewhat sharp radiating striae. The dorsal valve and intei-ior of 

 the shell have not been seen. 



