PELECYPODA—PRIONODESMACEA 



465 



Pierre : South Dakota, Colorado ; also in Montana of Wyom- 

 ing, Alberta, etc. 



279. 0. pellucida Meek and Hayden. Cretacic. 

 Similar to 0. mesentcrica in size, thinness, falcate outline, and 



marginal plications, but differs in its larger scar of attachment 

 which extends from the umbo to near the middle of the valve ; 

 also lacks the straight hinge line with ears characteristic of O. 

 mesenterica. 



Montana : Wyoming, South Dakota (Fox Hills), North Dakota 

 (Pierre), Montana (Pierre). 



280. 0. (Gryphaeostrea) vomer (Morton). (Fig. 619). 



Cretacic-Eocenic. 

 Lower valve deep. Upper valve marked with concentric plates. 



FlG. 619. Ostrea {Gryphceosti'ea^ vomer, a, upper valve; 6, c, lower valve. 



(Md. Surv.) 



New Jersey (Marshalltown-Vincentown), Delaware, Texas. 

 Eocenic : Maryland. 



281. 0. sellaeformis Conrad. (Fig. 620, a.) Eocenic. 

 Both valves deeply sinuous through vertical folding. 

 Eocenic : Maryland (Nanjemoy)-Alabama (Lignitic). 



282. 0. compressirostra Say. (Fig. 620, b.) Tertiary. 

 When young much like 0. scllceformis in form and plicate sur- 

 face. Differs when adult in lacking the sinuosity of 0. sellceformis. 



Eocenic : Maryland-Alabama. Oligocenic : Florida. Mio- 

 cenic : New Jersey-Florida. 



283. 0, trigonalis Conrad. (Fig. 621, a, b.) Eocenic-Pliocenic. 

 Lower valve few-ribbed. Upper valve flat. 



