X.] EXTINCT MAMMALS. 377 



ungulates. No trace of these singular artiodactyles has hitherto 

 been detected in the Old World. 



The Camelidce. seem to have been primarily a North American 

 family, which originated in the Sonoran region, and of which one 

 branch (Lama) subsequently migrated south, while the other 

 (Camelus) crossed Bering Strait into the Old World. In the 

 upper Pliocene there occurs Pliauchenia, with only three lower 

 pre-molars, and in the lower Pliocene Loup-Fork beds Procamelus 

 with four of these teeth ; while the earliest representative of the 

 family is Leptotragulus of the Uinta Oligocene. 



In the perissodactyle section of the same order the family 

 Titanotheriidce is mainly North American, although, as stated on 

 page 107, a representative of the typical genus Titanotherium has 

 been discovered in the Tertiaries of the Balkans. Titanotherium 

 includes huge rhinoceros-like animals, with low-crowned molar 



FIG. 8r. RIGHT UPPER MOLAR TOOTH OF Palaosyops, 



teeth of the type of those of Chalicotherium, and frequently having 

 the nasal region of the skull surmounted by large bony protuber- 

 ances. The genus is characteristic of the Uinta and the lower 

 division of the White River Oligocene. An earlier type of the 

 same family is typified by the smaller and less specialised hornless 

 animals from the Bridger Eocene, known as Palaosyops, which, 

 together with certain allied forms, constitute a peculiar sub-family 

 confined to America. This family, like the camels, appears there- 

 fore to have originated in the Sonoran region, whence a few 

 representatives wandered eastwards into the Old World. 



In the generalised ungulate sub-order termed Amblypoda, 

 of which the lower Eocene coryphodons were the earliest 

 representatives, North America possesses an absolutely peculiar 



