HISTORY OF THE PERISSODACTYLA 313 



unconformity, where the remains of ftitanotheres are so abun- 

 dant, successive changes may be observed. The species with 

 great "horns," rounded, flattened or triangular, are confined 

 to the upper levels ; in the middle section other species, some- 

 what smaller and with shorter "horns," are found, while in 

 the bottom levels the animals are much smaller and have still 

 smaller "horns." 



The Uinta ftitanotheres were much more numerous and 

 varied than those of the White River; in the upper part of 

 these beds are found two genera {\Diplacodon and ]Protitan- 

 otherium) which already had quite prominent bony protuber- 

 ances on the nose ; their canines were large enough to be of 

 value as weapons and the incisors were well developed and 

 functional. Evidently, there was a change here in the manner 

 of feeding, the front teeth were used for cropping and browsing, 

 a function which in the White River members of the family 

 must have been largely taken over by the lips and tongue, whUe 

 the growth of the horn-like protuberances on the skull rendered 

 the canines superfluous as weapons. This latter change is one 

 which recurs frequently in different phyla of the hoofed animals, 

 in which the earher and more primitive members had canine 

 tusks, and the later, more advanced representatives developed 

 horns, the tusks diminishing as the horns increased. While this 

 rule is a general one, it is not entirely without exceptions. 



In the lo\<i'er Uinta and in the Bridger the ftitanotheres 

 were extremely abundant and numerically they are the com- 

 monest of all fossils in those beds; no less than five series or 

 phyla may be distinguished, three of them being added in the 

 upper Bridger. The differences between the phyla, however, 

 principally concern the forms of the teeth and the shape of 

 the skull; in some the head was short and broad, in others 

 long and narrow, and in others again of medium proportions ; 

 some had broad and extremely low-crowned grinding teeth, 

 which in others were higher and more erect. But these are 

 matters of minor detail, useful as they are in pointing the way 



