MOLLUSCA — PELECYPODS 



227 



Exogyra (Fig. 96). Upper Jurassic-Cretaceous. 



Attached like Ostrea by the left valve but distinguished by the 



strongly twisted beak of this valve. Right or upper valve 



usually flat. Surface varying from nearly smooth to strongly 



A 



Fig. 96. — The oyster-like pelecypod, Exogyra arietina Roemer, from the Del Rio 

 clay of the Washita formation (uppermost Comanchean) of Texas. A , right vah^e 

 shown in place within the larger left valve. (After Hill.) B, outer view of a slightly 

 imperfect left valve. C, inner view of same ; ad., adductor muscle scar ; /., liga- 

 ment groove. Natural size. 



plicate. The name refers to the twisted beaks from the Greek 

 exo, out, + gyros, a curve. 



Fig. 97. — An oyster-like bed formed by Exoi^yra arietina in the aiuiiiit seas 

 covering much of Texas during uppermost Comanchean (Del Rio) time. See 

 Fig. 96. (X I) (From Hill.) 



