HISTORY OF THE AKTIODACTYLA 



377 



nostrils above the surface of the water. The tympanic bullae 

 (see p. 66) or bony chambers into which the ear-tubes opened, 

 were of relatively enormous size and added much to the unusual 

 appearance of the skull. The incisors were very small and the 

 grinding teeth narrow and completely hypsodont, this and 

 the ^Merychyus series being the only two phyla of the family 

 in which the hypsodont molar was fuUy acquired. The re- 

 mainder of the 

 skeleton differed 

 but httle from 

 the type common 

 to the whole 

 family, except 

 for a somewhat 

 shorter tail. 



The animals 

 of this series were 

 common in the 

 middle and lower 

 Miocene and in 

 the upper sub- 

 stage of the 



White River, but have not been found in the intermediate 

 John Day. This may have been a matter of geographical dis- 

 tribution, these creatures not extending west of the main 

 ranges of the Rocky Moimtains. In the upper White River 

 the genus ^Leptaiichenia is extremely common, but below that 

 level they suddenly and completely vanish and, as in the case 

 of the "fMerychyus phylmn, it is not yet practicable to deter- 

 mine the point in time or space of their branching off from the 

 main stem of the family. Were the foreodonts not entirely 

 confined to North America, we should, as a matter of course, 

 explain the seemingly sudden appearance of fLeptaiichenia as 

 due to inamigration, and it is entirely possible that the series 

 did actually originate in some part of North America which 



Fig. 200. — Skull of \Leptauchen%a nitida, upper White 

 River. 



