CRETACEOUS FOSSILS. 173 



shell, I am of the opinion that it is most probably not more than subgener- 

 ically distinct from Corhula proper. As I have elsewhere stated, we have 

 (at present) reason to believe that in this internal region of the continent, 

 the changes from marine conditions, first to comparatively large areas of 

 brackish-water, then to more restricted bodies of fresh-water, were so 

 gradual, as the continent was slowly rising at about the close of the Creta- 

 ceous, and the beginning of the Tertiary periods, that probably a few types 

 of MoUusca, originally exclusively marine in their habits, may have grad- 

 ually adapted themselves to a brackish-water habitat. 



The most characteristic specific features of this shell are its large size, 

 very nearly equal valves, gibbous anterior, attenuated or subrostrate poste- 

 rior extremity, and incurved, equal, and rather oblique beaks. In most 

 specimens, the valves seem to be almost exactly equal, though usually a 

 close examination reveals the fact that the right valve is a little larger than 

 the other; although the immediate apex of its beak curves in under that 

 of the left valve, and seems to be placed very slightly farther forward. 



I have been inclined to think that the species described from the Califor- 

 nia Cretaceous by Mr. Gabb, in vol. ii of the California Palaeontology, under 

 the name Corbula alceformis, may be somewhat nearly related to this species. 

 At any late, certain varieties of our shell seem to agree very closely with 

 Mr. Gabb's figure and description of his species. Most of our specimens 

 are more coarsely furrowed and ridged ; but, as already stated, they vary 

 greatly in this respect, so much indeed, that it is difficult to know how 

 far we can rely on this character in distinguishing Mr. Gabb's species from 

 our type, especially as he has figured only a single specimen. The fact, 

 however, that our shell is apparently only found associated with fresh-, or 

 perhaps in part, brackish-water types, while Mr. Gabb's came, if I am not 

 mistaken, from a marine deposit, would favor the conclusion that there 

 w^ere developed sufficiently marked differences to constitute specific, if not 

 more important, distinctions. 



Locality and position. — The original type-specimens of this species were 

 brought by Colonel Simpson's party from Sulphur Creek, near Bear River, 

 Wyoming. The Union Pacific Railroad there cuts directly through a small 

 ridge composed of the upturned strata of the estuary beds containing this 



