164 THE AGE OF MAMMALS 



the absence of upper cropping teeth, and in the presence of spear-hke 

 tusks and two pairs of prominent osseous horns. The body is intermediate 

 in proportion between those of the elephant and rhinoceros. The feet are 

 extremely short and broad. The brain (Fig. 71) is no larger than that 

 of a dog, out of all proportion to the body, and essentially of the archaic 

 type. These animals are very abundant both in the Upper Bridger and 

 Lower Washakie, and serve to tie these formations into a single life-zone. 



Of the creodont Carnivora the small hysenodont Sinopa is becoming 

 somewhat more rare. The large, powerful creodont Potriofelis of the 

 family Oxyaenidae, analogous in its proportions on a powerful scale to the 

 modern wolverine (Gulo) , and in its dentition to Hycena and Synoplotherium, 

 was capable of attacking the largest contemporary mammals. Here also 

 the skulking and swift-footed Mesonyx (or Dromocyon, Mesonychidse) is 

 represented by species of intermediate size (Fig. 52). Of the modernized 



Fig. 61. — The armadillo in North America. Skeleton of the Middle Eocene M etacheiromys 

 tatusia of the Bridger formation. In the American Museum of Natural History. 



ungulates the tapirs (Isedolophus) occur, but are rare. The horses (Oro- 

 hippus) as well as the primates (Notharctidse) are in a somewhat more 

 advanced stage of dental evolution than in the Lower Bridger levels. The 

 titanotheres now become polyphyletic through the appearance of three 

 additional phyla (Telmatherium, Manteoceras and Mesatirhinus) in ad- 

 dition to the extremely broad-headed Palceosyops Thus indications of at 

 least five phyla of titanotheres now occur, including long-headed forms 

 with more hypsodont teeth, and short-headed forms with more brachyo- 

 dont teeth. Of great interest is the development of rudimentary horns 

 on the forehead above the orbits, which appear as 'rectigradations,' in all 

 of these phyla. 



(2) Lower Washakie : Uintatherium Zone. — The Lower Bro^vn Beds of 

 the Washakie contain a fauna identical with that of the Upper Bridger 

 C and D, namely, Uintatherium, Palceosyops, Manteoceras, Mesatirhinus, 

 Notharctus, Hyrachyus, and Sinopa, in fact, the genera of the Lower Washa- 

 kie are almost without exception found in the Bridger and are represented 



