130 OAKBONIFEROUS PERIOD. 



c-ouvoxity from the umbo to the antero-hxteral margins about the same as 

 it is from side to side ; beak small, prominent, and incurved over that of 

 the dorsal valve ; mesial sinus broad, becoming obsolete near the middle of 

 the shell, and is entirely wanting upon the posterior portion. 



Surface marked by from fourteen to sixteen simple, angular, plications 

 upon each valve, with angular interspaces of similar width between them, 

 all of which are continuous from the front to the beaks ; about foiu- of these 

 plications are borne in the mesial sinus, and five of them upon the mesial 

 fold. 



Length, ten millimeters ; breadth twelve, millimeters ; height, seven 

 millimeters. 



This shell is more nearly related to R. Coojierensis Shumard from the 

 Subcarboniferous strata of Missouri than any other species with which I am 

 acquainted; but it differs in having a less number of plications, and in want- 

 ing the numerous filiform strise that mark the surface of that shell. 



Position and locality. — Strata of the Carboniferous period ; Old Potosi 

 Mine, Lincoln County, Nevada. 



Rhynchonella Wasatchensis White. 



Plafo IX, fig. 3 a, h, c, and d. 



Rhynchonella Wasatchensis White, 1874, Exp. & Surv. west 100th Merid., Prelim. Rep. 

 Invert. Foss., 19. 



Shell under medium size, sublenticular or subglobose ; valves nearly 

 alike in their convexity; length equal to, or slightly greater than, the 

 breadth ; antero-lateral and front margins sonKiwhat regularly rounded, the 

 front being slightly straightened or a little emarginate ; postero-lateral mar- 

 gins converging to the beaks at an obtuse angle. 



Dorsal valve broadly convex from side to side ; convexity from beak 

 to front regular but a little greater than it is transversely ; umbo prominent ; 

 beak strongly incurved ; mesial fold obsolete or wanting. 



Ventral valve strongly- and somewhat regularly arching from beak to 

 front ; convexity from side to side a little less than it is longitudinally and 

 about the same as the transverse convexity of the other valve ; beak some- 

 wliat prominent and incurved over the beak of the dorsal valve ; mesial 

 sinus obsolete or wanting. 



