PELECYPODA—PRIONODESMACEA. 



485 



nearly straight and sloping posteriorly. Shell truncated pos- 

 teriorly. Surface crossed by regular broad concentric ribs with 

 flat and equal interspaces. An angular ridge extends from the 

 beak to posterior basal margin. 



Hofsetown and Chico : California, Oregon. 



330. T. evansana Meek. Cretacic. 

 Somewhat similar to T. eufaulensis in outline, convexity, and 



strongly incurved beaks ; but its 

 beak is more elevated, its average 

 length is i| inches and height 

 1 1 inches, its posterior basal 

 margin is straight or slightly 

 convex, not concave, its ribs are 

 18 to 23 and proportionally 

 much narrower than the inter- 

 spaces and less curved than in 

 T. eufaulensis. 



Chico : California, Oregon. 



331. T. thoracica Morton. (Fig. 

 648.) Cretacic. 



Ribs nodose, about 15, the anterior ones curving strongly for- 

 ward. The posterior dorsal area occupied by 12 to 14 subangular 

 ribs. 



New Jersey (very abundant in Marshalltown ; also in Wenonah 

 and Navesink), Gulf region. 



332. T. eufaulensis Gabb. (Fig. 649.) Cretacic. 



FlG. 648. Trigonia thoracica. (After 

 Weller, N. J. Surv., Pal., IV.) 



Fig. 649. Trigonia eufaulensis. (After Weller, Pal. N. J., IV.) 



Drawn out posteriorly. Ribs angular, 12-14. 

 New Jersey (Merchantville-Wenonah), Gulf region. 



