464 THE MAMMALIA OF THE UINTA FORMATION. 



throughout by the presence of some form or other of the Dinocerata, and that this 

 extraordniary group of animals is entirely confined to this formation. 



A still more intricate and difficult problem is that with reference to the relations 

 of the various areas of Bridger deposits (apart from the Wind River series) to each 

 other. Of these areas there are three : (1) The Bridger basin west of the Green river ; 

 (2) the "Washakie basin east of the same river, and (3) a small area to the south- 

 east of the latter. Supposing the Bridger beds of the Washakie and Bridger basins 

 to have been deposited conformably in the same lake which laid down the Green 

 River series and to have been uplifted together with the Green River in a post-Bridger 

 upheaval, it is not a little remarkable that erosion should have removed the Bridger 

 from all parts save the middle of these two basins. The few observations which bear 

 upon this point in the way of the dips of the two formations combine to indicate that 

 the movement took place at the end of the Green River period, that the western lake 

 [^". e., the supposed extension of the Green River lake west of the Wasatch moun- 

 tains] was extinguished by this upheaval, and that the waters of the period formed a 

 lake of restricted area altogether within the basin of Green river. Even with this 

 supposition, which I conclude to be the most probable until it may be varied by 

 future evidence, there is left the shadow of a doubt, whether the three Bridger bodies 

 which appear upon our map — that of the Bridger basin, the Washakie basin, and the 

 region east of Vermilion creek — were parts of a continuous sheet, or whether they 

 themselves were areas of special lakes in the same general basin, but characterized 

 by great fauna resemblances." (King, TsTo. 6, p. 391).) 



The facts of stratigraphy, so far as at present known, leave this question an 

 open one, but the asseml>Iage of fossils seems to point to the conclusion that there 

 were at least two distinct lakes, and these not contemporaneous, but successive, a 

 conclusion which we have already indicated elsewhere (Osborn, No. 10, p. 13 ; Seott, 

 Xo. 15). The faunal lists of the two basins seem to show that the Bridger contains 

 slightly older deposits than the Washakie, though the two may be in part contem- 

 poraneous. Some of the differences to be noted are, no doubt, due in part to the 

 fact that the Bridger basin has been much more frequently and thoronghl}' explored 

 than the Washakie, and others probably to conditions of preservation, for within 

 the limits of the Bridger basin there are localities and strata which are especially 

 rich in certain soils of mammals which elsewhere arc much rai-er. As an example 

 of this may be mentioned the abundance of th(! lemuroids and other small mammals 

 at Twin Biittes. Future explorations may perhaps therefore diminish the number of 

 faunal differences between the two areas. 



A striking fact is the greater richness and variety of the forms found in the 



