24(3 UNITED STATES GEOLOGICAL SURVEY OP THE TERRITORIES. 



groove extends from the back part of the beaks, obliquely backward and 

 downward to near the middle of the anal border. On internal casts, a rather 

 distinct, broad, obtusely angular depression is seen just back of the beak of 

 each valve, apparently as though corresponding to a ridge or prominence on 

 the interior of the valves. The inequality of the valves results almost entirely 

 from the greater convexity and more elevated beak of the right valve, and 

 not from its margins extending beyond the edge of the other. 



Locality (Hid position. — Long Lake, near Fort Pierre, Dakota Territory; 

 from the Fox Hills group of the Upper Missouri Cretaceous series. 



Genus CORBULAMELLA, M. & H. 



Synon— Corbula t (sp.), Meek and Hayden (1856), Proceed. Acad. Nat. Sci. Philad., VIII, 84. 



Corbulamella, Meek and Hayden (1857), ib., 143. — Gabb, Synopsis Moll. Cret. Formation, 111. — 

 Stoliczka (1S70), Pala'out. Iudica, III, 37. 



Eli/m. — Corbula ; and lamella*, a little plate. 

 Type. — Corbula? gregaria, M. &• H. 



Shell small, inequivalve, subequilateral, trigonoid-subglobose ; right valve 

 more convex than the left; hinge of right valve, with a comparatively large 

 cartilage-pit under the beak, and just in front of this pit provided with a 

 conical tooth ; left valve with a tooth or cartilage-process fitting into, and 

 partly filling the pit of the other valve ; interior of each valve with a very 

 prominent, spoon-shaped process, connected with, or bearing the posterior 

 muscular scar; pallial line apparently a little sinuous. 



The type ot this genus is a very small shell, having apparently precisely 

 the form and hinge of Corbula, but differing remarkably in the possession 

 of a very prominent, spoon-shaped plate or process, connected with the pos- 

 terior muscular scar of each valve. This process is somewhat similar to that 

 seen in Cardilia, Deshayes, but more prominent and rounded below. The 

 nature of the hinge and the inequivalve character of the shell, however, 

 readily distinguish it from that genus. 



At first, 1 was in doubt whether or not this genus has an internal carti- 

 lage as in Corbula ; but a single specimen, since examined, with the two 

 valves united, and the cardinal margins exfoliated, or worn away so as to 

 expose the hinge from above, shows that the tooth or process of the left valve 



* It. will be seen that Corbulalamella would have been the correct orthography of this name ; but 

 as it ■was at first inadvertently printed as above, I do not think it a matter of sufficient importance to 

 rendei the change now desirable, especially as it would make the name inconveniently long. 



