192 STANTON—MOLLUSCAN FAUNULE. [April 3, 
marine Cretaceous. That the formations thus assigned in many 
cases include the equivalent of the true Dakota is very probable, 
but that they may also include other horizons, laid down in the long , 
interval between the Jurassic and the Upper Cretaceous, is equally 
probable. In the Yellowstone Park and adjacent areas the sup- 
posed Dakota includes bands of impure limestone filled with fresh- 
water shells not found elsewhere. Among them is a Unio repre- 
sented by rare casts and fragments, but the most of them are 
simple gastropods, the most common of which I have described as 
Gontobasts ? pealet, G.? increbescens and Amnicola ? cretacea.._ The 
beds referred to the Dakota near Great Falls, Montana, already 
mentioned, contain another assemblage of three or four species of 
fresh-water mollusca not known elsewhere. None of these sup- 
posed Dakota beds in the region just mentioned yielded the char- 
acteristic Dakota flora. 
The Bear River formation of western Wyoming is the last one to 
be considered in this connection. It consists of a great thickness 
(as much as 4000 feet in some sections) of conglomerates, sand- 
stones and shales, having a large and peculiar fresh-water fauna. 
Its principal known area extends from the neighborhood of Evans- 
ton, on the Union Pacific Railroad, northward near the western 
boundary of Wyoming for more than a hundred miles. Origin- 
ally it was assigned to the Tertiary, afterward to the Laramie, or 
uppermost Cretaceous. It is now known to lie between the Fort 
Benton and the marine Jurassic,” but just how muck or what part 
of this interval it represents is not positively known. Some of the 
conglomerates associated with the fossiliferous beds have probably 
been mapped as Dakota by the early surveys. ‘The occurrence of a 
few undetermined dicotyledonous plants of modern type in the forma- 
tion favor its assignment to the Upper Cretaceous. The fauna is not 
closely related to any other known on this continent, as all the species 
are restricted to it, and at least two of the gastropod types are so 
peculiar that they have been described as new genera. The entire 
fauna has been reviewed and figured by Dr. C. A. White,* who has 
made detailed comparisons with other non-marine faunas. One of 
the most striking of the common fossils of the fauna is Pyrgulifera 
1 Monog. U. S. Geol. Surv., Vol. XXXII, Pt. 2, pp. 632, 633, 1899. 
2See Stanton: «The Stratigraphic Position of the Bear River Formation,” 
Am. Four. Sci., 34 ser., Vol. XLIII, pp. 98-1 a5 1892. 
. > Bull, U. S. Geol. Surv., No. 128. 
rr. 
