530 



NORTH AMERICAN INDEX FOSSILS. 



with 10 to 12 radiating ribs and with very regular, sharply defined 

 and closely arranged concentric ridges. In form quite similar to 

 P. marylandica but it is much smaller (average length 1.2 inches ; 



height 0.8 inch ; convexity 0.6 inch), 

 and the surface markings are more 

 prominent. 



Benton : Kansas, Montana. 

 464. P. occidentalis Morton. 



Cretacic. 

 Surface marked with 25 to 30 radi- 

 ating, somewhat irregular, and wavy 

 ribs, closest together on middle of 

 shell. 



Ripleyan of New Jersey (ClifTwood- 

 Woodbury) ; Gulf region, Arkansas. 

 465. P. marylandica Conrad. (Fig. 719) Eocenic. 



Fig. 718. Pholadomya sandi- 

 sabce. (After Meek, Mex. Bd. 

 Surv. ) 



FlG. 719. Pholadomya marylandica, left valve. ( Md. Surv. 



Length about 3 inches, very fragile. 

 Aquia : Maryland. 



CXXVli. Phenacomya Dall. 

 Equivalve, thin, marked with feeble radii. Anterior end more 

 or less expanded and truncate. Posterior end attenuate and 

 smoother. Eocenic. 



