INVERTEBRATE PALAEONTOLOGY. 521 



on the Union Pacific Railroad, Wyoming, seem to be in all respects (indis- 

 tinguishable from tins species by any external characters. They do not 

 show the hinge, but I am strongly inclined to believe them identical with 

 this species. It is a coincidence also worthy of note here, that at this Black 

 Butte station, and in the same bed from which the specimens here mentioned 

 were obtained, the remains of a Dinosaurian were found; while Saurian 

 remains of Cretaceous affinities also occur in the same beds with the type- 

 specimens of the species here described, in the Upper Missouri country. 



I also have now before me numerous fine specimens of a very similar 

 form, from a much lower horizon than the Black Butte station, some twelve 

 miles farther westward on the Union Pacific Railroad, near Point-of-Rocks 

 Wyoming. The only differences that I can see between these specimens and 

 the species here under consideration are, that they have the. beaks somewhat 

 more elevated and gibbous, and the substance of the shell thinner. With the 

 exception of the latter character, a slightly more forward curvature of the 

 beaks, and a very little greater proportional length of the valves, this last 

 mentioned Wyoming form is almost as nearly related to the first of the fol- 

 lowing-described species. It holds a position in the Upper Cretaceous * 



Locality and position. — The type-specimens of C. cytheriformis, came 

 from the Bad Lands of Judith River, on the Upper Missouri, in Montana. 

 Others in the collection are from the mouth of Muscleshell River; from near 

 Port Benton; and from one hundred and twenty-five miles below the same. 

 The geological horizon is the same as the last — that is, probably the latest of 

 the Cretaceous, or possibly the oldest Eocene. 



Corbie ii la occidentalis, M. &H. 



Plate 40, figs. 6, a, b, c. 



Cyrcna occidentalis, Meek and Hayden (1856), Proceed. Acad. Nat. Sci. Philad., VIII, 116. 

 Corbicula occidentalis, Meek (1860), ib., XII. 432. 



Shell attaining a moderately large size, subtrigoual in form, with height 

 and length about equal, rather gibbous, thick and strong; dorsal margins 

 sloping nearly equally from the beaks, the anterior slope being almost straight, 

 or slightly concave in outline, and the posterior a little convex ; anterior and 

 posterior margins rounding into the base, or the latter sometimes slightly 



*Tbis form from near Point-of-Rocks, Wyoming, if found to be specifically distinct from tlo 

 others, might lie designated as C. Wyomingensis. 

 06 H 





