lO 



E. M. MUSEUM MEMOIRS. 



liarities of species and genera are of diminished value when isolated from an 

 investigation of their natural and physical environment. We cannot grasp the 

 meaning of the peculiar dentition, of the anomalous features of the skull, or, 

 what is more important, of the modifications of the ankle-joint and feet ; in short, 

 of the adaptability of the skeleton to certain functions, unless we first inquire into 

 the nature of the moist and semi-tropical country which formed the habitat 

 of these Eocene animals. However vague and unsatisfactory this inquiry be, 

 it affords a collection of additional facts upon which to advance. 



Clarence King in his recent work' has given an original and interesting 

 account of the early geological history of this region. And this memoir upon 

 the Dinocerata cannot be more appropriately introduced than by a r^sum^ of 

 his narrative, accompanied by several observations of our own. 



At the close of the Cretaceous period, the country lying between the pres- 

 ent Rocky Mountain system and the Wahsatch range lay open to the sea on all 

 sides. By the elevation of the land which now forms the summit of these ranges, 

 a great area became enclosed on the East and West, and the convergence of 

 the ranges somewhat in crescent shape formed a northern shore. To the 

 South the waters extended into Colorado and New Mexico. The discoveries 

 of Hayden and Marsh have confirmed and increased this southerly extension. 

 Its exact limits are not yet ascertained, nor do they, although interesting, im- 

 mediately belong to the present subject. 



Thus, at the beginning of the Eocene an inland sea was formed, almost as 

 broad as Lake Erie, enclosed on the North by the spurs of the great system 

 of hills which formed the eastern and western barriers. These spurs are repre- 

 sented by the Bear River, Wind River, and Sweet Water mountain systems of to- 

 day. On the southwestern shore was a great island, the Uinta chain, also of 

 Secondary rocks, lying in an east and west direction. This inland lake therefore 

 filled the basin which is at present drained by the head-waters of the Green 

 and Colorado river systems, and the Green River now cuts through the eastern 

 base of the Uinta range, marking the central point of southern outlet probably 

 from the earliest enclosure of the basin. These waters, which King has named 

 the Ute Lake, gradually lost their saltness by the drainage from the hills sur- 

 rounding them, while the mouth, or southern outlet, was slowly losing its direct 

 communication with the sea. This change from marine to lacustrine con- 

 ditions was preceded by the extinction of many of the reptiUan types of the 

 Cretaceous. 



Underlying the waters of the lake, somewhat tilted by the upheavals which 

 formed the shores of the basin, were the upper members of the Cretaceous 

 series, the Laramie beds. Here and there also were islands of earlier Second- 



' U. S. Geol. Explor. of Fortieth Parallel. Vol. I. Systematic Geology. 



