462 



NORTH AMERICAN INDEX FOSSILS. 



268. 0. cretacea Morton. Cretacic. 



Irregularly ovate, non-plicate. Lower valve moderately con- 

 vex ; upper valve nearly flat. Surface marked with concentric 

 undulations. Height about -| inch ; width slightly greater. 



FlG. 614. a, b, Ostrea lugubris, lower and upper valves ; c, d, O. congesta, interior of 

 upper valve slightly enlarged and a group of attached lower valves. (After Stanton.) 



Ripleyan of New Jersey (ClirTwood), Georgia, Alabama, 

 Arkansas. 

 269. 0. denticulif era Conrad. (Fig. 615.) Cretacic. 



Flattened, thick. Surface obscurely striate. Margin with tooth- 

 like crenulations. 



Ripleyan of New Jersey (Woodbury), Mississippi, Tennessee. 



Fig. 615. Ostrea denticulif era, Conr. (N. J. Pal., I.) 



270. 0. subspatulata Forbes. Cretacic. 



Outline somewhat wedge-shaped with rounded margins. Shell 



widest below the middle whence it narrows gradually upwards. 



Lower valve strongly arcuate longitudinally. Surface marked 



