238 GEOLOGICAL SURVEY OF THE UNITED STATES TERRITORIES. 



small, external; cartilage-ossicle small, subcircular ; sinus of pallial line 

 shallow. 



In several respects, this genus seems to be related to the CorbuUdcc, in 

 which some conchologists range it. The presence of a free ossicle in the 

 cartilage of these little shells, however, as well as some of their other char- 

 acters, appears to indicate relations to the Anatinidae. 



The genus Necera was probably introduced toward the latter part of the 

 Cretaceous epoch. A few species, however, have been referred to it from 

 the Jurassic rocks; but they are only known, I believe, from external charac- 

 ters, and probably belong to the genus Leda, or some other group. Several 

 species are known from the Middle and Upper Cretaceous rocks, and the 

 genus was more abundantly represented during the deposition of the Tertiary 

 formations. It is also represented by a number of species in the seas of the 

 present epoch, where it probably attains its maximum development. The 

 recent species are widely distributed, and inhabit deep waters. They are 

 found on the coasts of China, Borneo, Chili, Madeira, New Guinea, Norway, 

 Britain, &c, as well as in the Mediterranean. 



Hfpaera ventricosa, M. & H. 



Plato 30, figs. 3, o, b, c, d, c. 



Corbula ventricosa, Meek and Hayden (1856), Proceed. Acad. Nat. Sci. Philad., VIII, 83. 

 Necera ventricosa, Meek and Hayden (May, 1800), ib., XII, 185. 



Shell small, nearly or quite ecpjivalve, rather thin, very ventricose in the 

 anterior and central regions ; anterior margin somewhat narrowly rounded ; 

 base very deeply rounded toward the front, contracted behind ; posterior side 

 longer than the other, narrow, compressed, and rostriform ; dorsum sloping 

 gradually with a concave outline behind the beaks, declining more abruptly 

 in front; beaks prominent, apparently equal, not oblique, located a little in 

 advance of the middle; pallial border smooth; surface marked by rather dis- 

 tinct, concentric striae. 



Length, 0.32 inch ; height, 0.20 inch ; convexity, 0.14 inch. 



A right valve of this species in the collection shows the cavity under 

 the beak for the reception of the cartilage to be very small and narrow. Just 

 behind this cavity, there is a prominent, horizontally-compressed, curved 

 tooth, which projects distinctly beyond the cardinal margin. The posterior 

 muscular impression, which is narrow, somewhat irregular, and quite strongly 



