506 



NORTH AMERICAN INDEX FOSSILS. 



short, about one half the size of the anterior. Anterior ear rather 

 large, separated from the body of the shell by an angular sinus 

 about half as deep as the length of the ear. Radiating striae very 

 fine and regular, increasing by intercalation. Concentric striae 

 fine and regular, closely arranged. Average height and breadth 

 each i^ inches. Differs from P. platessa in being proportionally 

 broader, with shorter ears and finer radiating striae. 

 Upper Jurassic : South Dakota, Wyoming, Utah. 



389. P. (Camptonectes) extenuatus Meek and Hayden. 



Jurassic. 



Differs from P. bellistriatus in its smaller size, more convex 

 valves, more erect form, and height slightly greater than width. 

 Umbonal angle about 105 °, height | inch ; breadth T 9 g inch. 



Upper Jurassic: South Dakota (Sundance), Wyoming (Shirley), 

 Utah. 



390. P. (Camptonectes) pertenuistriatus Hall and Whitfield. 



Jurassic. 



Differs from P. extenuatus in its 

 more attenuated beak (umbonal angle 

 about 90 ), flattened valves, finer 

 striae, and more elongate form. Aver- 

 age height 1 inch ; breadth -| inch. 



Montana, Wyoming. 

 391. P. (Camptonectes) platessa 



White. (Fig. 6yy.) Cretacic. 



Surface marked with fine radiating 

 striae which at the sides of the shell 

 recurve. 



Coloradoan : Utah, Arizona. 



Fig. 677. Camptonectes platessa, 

 right valve. (After Stanton. ) 



C. Entolium Meek. 



A thin Pecten, with nearly equal ears which diverge at a sharp 

 angle above the beaks. Surface nearly smooth. Mississippic 

 Cretacic. 

 392. P. (Entolium) aviculatus (Swallow). (Fig. 678.) 



Mississippic and Carbonic. 



Surface marked with fine, close concentric striae and occasional 

 faint traces of radiating ones. 



