PELECYPODA—PRIONODESMACEA. 473 



A. Surface smooth or nearly so I. 



I. Larger valve marked with broad furrow 298. £. suborbiculata. 



I. Larger valve not furrowed 300. E. Iceviuscula. 



B. Surface marked with prominent concentric lamellae II. 



II. Shell large 301. E. ponderosa. 



II. Shell small 297. E. arietina. 



C. Surface marked with distinct radiating ribs or wrinkles III. 



III. Small 1. 



I. Nearly flat 2 9$ a - & texana var. weatherfordensis. 



1. Convex a. 



a. Shell with obtusely rounded umbonal ridge 299. E. columbella. 



a. Shell with angular umbonal ridge 296. E. plexa. 



III. Large 2. 



2. Valves very unequal, the lower very convex, the upper nearly flat. 



302. E. costata. 

 2. Valves subequal, flattened 295. E. texana. 



295. E. texana Roemer. (Fig. 631-632.) Comanchic 



Valves subequal, thick. Surface marked with unequal radiat- 

 ing ribs. 



Widely distributed in the Comanchic of Texas, being especially 



Fig. 631. Exogyra texana. Opposite valves. (After Hill.) 



abundant in the Fredericksburg. Also in Fredericksburg and 

 Washita of Mexico. 



295a:. E. texana var. weatherfordensis Cragin. Comanchic. 



Small, flattened, with somewhat semicircular outline. Lower 



