1874.] 247 [Rathbun. 



which I have been able to compare it to merit description. It is 

 readily distinguished from Leda Swiftiana, above described, by its 

 more elongate form, less prominent valves, and by the posterior por- 

 tion of the hinge descending less rapidly. 



From the cretaceous bed at Sao Jose, Pro v. of Pernambuco, Bra- 

 zil. A few specimens only have been obtained. 



Family ARCID^. 



Area Orestis sp. nov. 



Shell of moderate size, elongate, somewhat compressed, and with 

 the vertical axis nearly two-thirds the antero-posterior. In outline it 

 is. subelliptical, the height being greatest near the middle, but not 

 varying much throughout the length of the shell. Beaks prominent, 

 rounded, not incurving or inclining forward very strongly. Their 

 distance from the anterior margin is much greater than one-third the 

 length of the shell. 



The hinge is equal, to two-thirds the length of the shell or slightly 

 more. The anterior extremity is not so high as the posterior and is 

 regularly rounded, the curve of the anterior margin continuing regu- 

 larly into that of the ventral. The latter is slightly rounded and de- 

 scends gradually in extending backward. The posterior margin is 

 slightly oblique and rounded. 



The valves are most prominent in the umbonal region, but become 

 flattened in the lower two-thirds. The surface rounds gradually into 

 the posterior slope, which forms a slight sigmoidal curve in descend- 

 ing toward the hinge. In no part is the shell angular. 



The markings of the shell consist of small, rounded, or slightly an- 

 gular, radiating raised lines or plications, separated by narrower, sub- 

 angular interspaces. The lines curve slightly in extending from the 

 beak to the margin. On the posterior slope they are very fine and 

 thread-like. The concentric lines are very small and numerous. 



Length of shell, 32 mm.; height, 21 mm.; depth, of each valve, 6 mm. 



Only a single specimen, a left valve, was obtained and that is much 

 exfoliated, more especially near the beak. The internal characters 

 are not exposed. This form is readily distinguished by the slight 

 prominence of its valves, and by the surface being everywhere with- 

 out abrupt curves. The markings are also quite simple. 



From the cretaceous bed at Maria Farinha, Pro v. of Pernambuco, 



