160 THE AGE OF MAMMALS 



cate mud beds, or shallow water conditions, in which the remains of uinta- 

 theres and rhinoceroses are occasionally found as if they had been mired 

 in a standing position (Hyrachyus has been found in this way). Occasion- 

 ally, however, the entire region must have been flooded for long periods, 

 because the careful researches of Granger and Matthew reveal the existence 

 of successive 'white layers' (Fig. 58) of great horizontal extent, largely 

 calcareous, or composed of shallow limestone containing shells {Goniobasis, 

 Pkmorhis), as well as weatherworn jaws of small mammals. In their 

 harder constituency these layers form the caps of the great benches or 

 steppes which subdivide the grand Bridger Formation into successive 

 layers, A — F. These hard layers also serve to mark off the Bridger into 

 faunal levels. In general on the lower levels we find smaller mammals 

 in lower stages of evolution, while on the higher levels we find larger mammals 

 in a more advanced stage. There is thus a general progression and ad- 

 vance of mammalian life from below upward, and secondly a gradual 

 change in the character of the fauna, partly due to extinction and partly 

 to invasion of the Bridger from the surrounding country. 



Washakie. — The Washakie lies fifty miles east of the Bridger, a for- 

 mation occupying an area of over 300 square miles, chiefly composed of 

 volcanic material and subdivided into the Lower Washakie (Brown Beds, 

 Uintatherium Zone, 260 feet in thickness) and Upper Washakie (Green 

 and Gray Beds, Eobasileus Zone, 380 feet in thickness). First men- 

 tioned by Hayden in 1869,' it is famous through the successive explora- 

 tions of Cope, Marsh, Scott, Osborn, and finally of Granger,^ who has 

 solved its geologic and faunal characters. 



Fish and Reptile Fauna 



In the stream channels of the Bridger have been found remains of 

 several species of bowfins (Amiidae) and garpikes (Lepidosteidae) as well 

 as of catfishes or siluroids (Rhineastes) . From the Lower Bridger are 

 also described three species of snakes. The crocodiles were numerous and 

 diversified, including Crocodilus as well as the diminutive Limnosaurus 

 with laterally compressed teeth. Several species of Lacertilia (Glypto- 

 saiirus) have been described. All these partially known reptiles give us 

 hints as to the Floridian or south temperate conditions of climate, and the 

 great abundance of aquatic life. We may picture partly open, partly forested 

 country, somewhat similar to the bayou region of the Mississippi Delta. 



More direct information is afforded from Dr. Hay's monographic 

 studies on the Testudinata.^ We might expect to find here representa- 



1 Hayden, op. ciL, 1869. 



- Granger, W., Faunal Horizons of the Washakie Formation of Southern Wyoming. 

 Bull. Amer. Mus. Nat. Hist., Vol. XXVI, no. 3, Jan. 19, 1909, pp. 13-23. 



^ Hay, O. P., The Fossil Turtles of North America. Publ. Carnegie Inst., Washington, 

 no. 75, 4to, 1908. 



