508 



NORTH AMERICAN INDEX FOSSILS. 



rather flat. Ears subequal. Ctenolium absent ; byssal notch in- 

 conspicuous or absent. Jurassic-Recent. 



395. Pecten (Amusium) alabamensis Aldrich. Eocenic. 

 Very small. Surface of right valve nearly smooth ; left with 



a few radial threads crossed by distant concentric lines. 

 Midwayan : Alabama, Arkansas. 



396. P. (Amusium) mortoni Ravenel. (Fig. 680.) 



Miocenic-Recent. 

 Thin. Inside of valves marked with 18 to 24 radiating double 

 ribs. 



Miocenic : Atlantic coast. Also Recent : Gulf of Mexico. 



Fig. 680. Amusium mortoni, right valve, X h (Md. Surv. ) 



OIL Plicatula Lamarck. 



Inequivalve, nearly equilateral, compressed. Surface bearing 

 coarse, radial, and often divaricate ribbing. Teeth long, shallow, 

 and crenulate, diverging at a sharp angle. Muscle scar single, 

 excentric. Shell sessile. Triassic-Recent. 

 397. P. dentonensis Cragin. Comanchic. 



Rounded, subtriangular. Surface marked with numerous, 

 crenulated, and often spine-bearing ribs with intercalated shorter 

 and finer ones. 



