346 LAND MAMMALS IN THE WESTERN HEMISPHERE 



Bridger stage, but was not, so far as is known, represented 

 in any of the subsequent stages, and was made up of a single 

 genus {'\Colonoceras) which had a small pair of dermal horns 

 upon the nasal bones. In other respects, it was like ]Hyra- 

 chyus. It is surprising to find that the horned series should 

 have so speedily died out, while the defenceless forms not only 

 persisted, but actually became more defenceless through the 

 reduction of the canine tusks. A priori, one would have ex- 

 pected the opposite result, but the key to the enigma is 

 probably to be found in the more perfect adaptation of the 

 surviving kinds to swift running. 



The second subdivision (fAmj^nodontinse) of this family 

 contains a series of animals which developed in a very divergent 

 fashion and went to quite the opposite extreme from the 

 cursorial fhyracodonts, resembling the latter (aside from the 

 fundamental characteristics common to all rhinoceroses, in 

 the broadest sense of that term) only in the pattern of the molar 

 teeth and in the absence of horns. The terminal member of 

 the famynodont series was a White River genus {'\Metamyno- 

 don) of which the remains have been found almost exclusively 

 in the consolidated and cemented sands filling the old river- 

 channels of the middle substage of the White River beds. 

 This fact, together with certain structural features of the skull 

 and skeleton, leads at once to the suggestion that these ani- 

 mals were chiefly aquatic in their habits and somewhat like 

 hippopotamuses in mode of life. '\Metamynodon was quite 

 a large animal, the heaviest and most massive creature of its 

 time, after the disappearance of the giant ttitanotheres, but 

 was low and short-legged. 



The true rhinoceroses, save those which, like the existing 

 African species, have lost all the front teeth, all agree in the 

 peculiar differentiation of the incisors, which was fully de- 

 scribed in the preceding section of this chapter. The fhyra- 

 codonts had a second scheme, the incisors and canines being 

 all similar in shape, small, pointed and recurved, while still 



