PELECYPODA—PRIONODESMACEA. 



407 



Subglobose. Its internal molds differ from those of C. tippana 

 in the more rounded outline and usually larger size. 



Ripleyan of Arkansas, Texas, New Jersey (characteristic espe- 

 cially of the Merchantville, also occurs in the Navesink). 



115. C. truncata Gabb. Cretacic. 

 Differs from C. gigantea in the narrower and more pointed and 



upright, nearly central beaks. 

 Chico : California, Oregon. 



116. C. gigantea Conrad. (Fig. 525.) Eocenic. 

 Varying much in size. Surface covered with numerous 



radiating striae and concentric growth lamellae. 



Fig. 525. Cucullcea gigantea, left valve from within and wi 



(Md. Surv.) 



Throughout the Eocenic of Maryland and Virginia ; Lignitic of 

 Gulf states. 



XXXIV. Trigonarca Conrad. 



Differs from Cucullcea in its somewhat trigonal form, obliquely 

 truncated posterior margin, and somewhat angular posterior basal 

 extremity. Hinge curved. Teeth radially arranged. Hinge 

 area divaricately furrowed. Cretacic and Tertiary. 



117. T. obliqua Meek. (Fig. 526, #-*.) Cretacic. 



Beaks small, depressed. Posterior umbonal slope prominent. 

 Both muscle impressions are bordered by elevated ridges. 



Coloradoan : Colorado (Pugnellus sandstone), Utah. 



XXXV. Breviarca Conrad. 

 Small, suborbicular, with more or less rounded extremities. 

 Surface nearly smooth or marked only with fine radii. Hinge 



