300 THE AGE OF MAMMALS 



and flat; the upper tusks are directed downward and outward, and orna- 

 mented with a ribbon-hke band of enamel; the lower tusks still retain 

 the horizontally flattened form characteristic of Palceomastodon, and the 

 degree of attrition indicates that they were in constant use in feeding. The 

 lower jaws are still elongate. P>om the parts of a skeleton attributed to 

 one of these animals, we may estimate the height as 5 ft. 10 in. at the 

 withers. The trilophodont molars indicate that this animal may have 

 been in the same line of descent as the great Mastodon americanus of the 

 Pleistocene. 



The teleocerine rhinoceroses at this stage are represented by the species 

 T. crassus, which is somewhat less hypsodont than the T. fossiger of the next 

 stage. Similarly the aceratheres {Aphelops hrachyodus) have not yet at- 

 tained the final stage of evolution. Two of these animals from the Flint 

 Creek beds of Montana^ (A. jnontanus, A. ceratorhinus) exhibit dolicho- 

 cephalic skulls, long and slender nasals, sometimes with small terminal 

 horn rugosities (A. ceratorhinus), brachyodont teeth, limbs relatively long 

 and slender. These proportions are in wide contrast with those of the 

 broad-skulled, short-footed contemporary Teleoceras of Montana, which 

 exhibits short nasals with a small laterally compressed terminal horn (fT. 

 crassus). 



The tapirs were very rare. 



The horses present a very high degree of adaptive radiation, fitted to 

 the diversified feeding grounds of the plains region and to the forests and 

 thickets bordering the streams. All of these horses exhibit pits or depres- 

 sions on the side of the face in front of the eyes, a character which points 

 away from rather than toward the ancestors of Equus. 



The oreodonts include the persistent Miocene genera Merychyus and 

 Merycochwrus, and in the Madison Valley beds of Montana there appears 

 the remarkable Pronomotherium, an extremely specialized brachycephalic 

 oreodont, with receding nasals, indicating in the plainest manner the pos- 

 session of a large proboscis.^ Even in Merycochoerus the face is greatly 

 shortened and probably supplied Avith a flexible upper lip and much shorter 

 jaw than the Upper Oligocene Promeri/cocAceru.s. The abbreviation of the 

 skull and adoption of fluviatile habits appear to have been features of the 

 closing chapter of the oreodont evolution. 



Among the camels Procamelus is readily distinguished from the Middle 

 Miocene Protolahis by the loss of two upper incisor teeth, a marked approach 

 to Camelus. The contemporary Pliouchenia is distinguished by the loss of 

 some of its premolar teeth, thus approaching the llamas, or South American 

 camels (Auchenia), in which the premolars are more reduced than in the 



' Douglass, E., New Vertebrates from the Montana Tertiary. Ann. Carneg. Mus., 

 Pittsburg, Vol. II, no. 2, 190.3, pp. 145-200. 



- Douglass, E., Promerycochcerus and a New Genus of Merycoidodonts with Some Notes 

 on Other Agriochoeridae. Ann. Carneg. Mus., Pittsburg, Vol. IV, No. 2, 1907. 



