474 NORTH AMERICAN INDEX FOSSILS. 



valve usually marked with plications which are delicate on the 

 posterior slope and somewhat tuberculate on the anterior. 

 Trinity of Texas. 



296. E. plexa Cragin. Comanchic. 

 Lower valve divided into an anterior and a 



posterior slope by an angular ridge. Both 

 slopes usually marked with radiating lines or 

 wrinkles. 



Comanchic : Texas (Fredericksburg and 

 Washita). 



297. E. arietina Roemer. (Fig. 633.) 



Comanchic. 



Larger valve inflated, tortuous. Smaller 

 valve flat, a plane spiral. Without plications. 

 Washita : Texas, Mexico. 



298. E. suborbiculata (Lamarck). (Fig. 

 635, c.) Cretacic. 



Fig. 632. Exogyra Lower valve marked with a broad furrow, 



texana. The two valves .. , .. , r . . . 



, ,, t extending obliquely from the umbo to the 



in conjunction. ( After *> -i j 



Hill.) postero-basal margin. Surface smooth. 



Benton : Colorado (Pugnellus sandstone). 

 299. E. columbella Meek. (Fig. 634, a-c.) Cretacic. 



Small, left valve marked by radiating ribs. Right valve flat, 

 oval and smooth. 



Colorado : New Mexico, Texas (Benton) ; also Dakota of Texas. 



FlG. 633. Exogyra arietina. (After Hill.) 



300. E. laeviuscula Roemer. (Fig. 634, d.) Cretacic. 



Both valves smooth except for concentric growth lines. Smaller 

 valve flat and nearly circular. 



Very common in the Austin of Texas ; also Benton of Kansas 

 and Utah. 



