PELECYPODA—PRIONODESMACEA. 



443 





Shell gibbous, obliquely subrhombic in outline. Beaks nearly- 

 terminal. Concentric undulations strong, subangular. 

 Benton : Kansas, Montana. 



212. I. gilberti White. (Fig. 581, &, c.) Cretacic. 

 Surface of each valve marked with a poorly defined median fur- 

 row and with extremely prominent concentric wrinkles. 



Benton of Kansas and approximately the same horizon in Utah. 



213. I. umbonatus Meek and Hayden. (Fig. 582, b.) Cretacic. 

 Left valve very convex with strongly incurved beak ; height 



more than one third greater than the antero-posterior diameter. 

 Right valve subcircular, nearly flat ; beak rather oblique ; surface 

 marked with concentric undulations. 



Benton : Kansas, Montana. Niobrara : Texas. 



214. I. labiatus Schlotheim. (Fig. 583.) Cretacic. 

 Outline obliquely elongate. Surface marked with concentric 



undulations. Average length about 

 4 inches ; breadth at right angles 

 to the greatest length 2 inches. 



Coloradoan throughout the 

 Plains and Rocky Mountain region. 



215. I. deformis Meek. (Fig. 



582, a.) Cretacic. 



Beak moderately prominent, sit- 

 uated between the middle and the 

 anterior extremity of the hinge. 

 Surface with strong regular or very 

 irregular concentric undulations be- 

 coming abruptly smaller on the 

 umbo where their curves indicate a 

 greater obliquity of the young 

 shell. Shell thick, prismatic structure coarse. 



Very abundant in the Niobrara of the Rocky mountains and 

 Plains. 



216. I. altus Meek. Cretacic. 

 Differs from /. fragilis in its larger size (about 6 inches high, 



hinge length 2 inches), more concentric undulations, though ob- 

 scure, and faint traces of radiating markings. 



Fig. 5^3- Inoceramus labiatus, 

 right valve. (After Stanton, U. S. G. 

 S., Bull. 106.) 



