476 



NORTH AMERICAN INDEX FOSSILS. 



301. E. ponderosa Roemer. (Fig. 635, a, b.) Cretacic. 

 Left valve marked with strong concentric lamellae, fine concen- 

 tric striae and obscure radiating ribs. Right valve with horizontally 

 spiral umbo ; concentrically laminated. 



Ripleyan of New Jersey (Marshalltown), Alabama, Arkansas. 

 In the Coloradoan of Utah, Texas (Austin) and Mexico. 



302. E. costata Say. (Fig. 636.) Cretacic. 



Fig. 636. Exogyra costata. Lower valve, X Yz- (After White.) 



Differs from E. ponderosa in being strongly costate and in lack- 

 ing the lamellose extensions. 



New Jersey (especially characteristic of Navesink ; also in Red 

 Bank and Tinton), Delaware, Maryland (Monmouth) and Gulf 

 region to Texas and San Luis Potosi. 



LXXXII. Amnigenia Hall. 

 Equivalve, very inequilateral, subelliptical, with very short an- 

 terior end and prolonged and wider posterior end ; both extremi- 



