248 BIOLOGICAL LECTURES. 



characteristic pad of the modern tylopodan foot was not de- 

 veloped till a later time. 



While there has been no dispute that Poebrotherium is the 

 White River representative of the main line of tylopodan 

 descent, there has been no such agreement concerning the 

 other selenodonts of that age, and this for the reason that 

 nothing has been known of their ancestry, and that among 

 existing mammals none could be selected as being a descendant 

 of any of these genera ; hence almost every conceivable opinion 

 as to their systematic position has been held and defended. It 

 is upon this very problem that the newly discovered Uinta forms 

 shed such welcome light, and they render it exceedingly prob- 

 able that all the strictly indigenous North American selenodonts 

 are branches of the great tylopodan stem. 



Paradoxical as this conclusion may appear to be, I believe 

 that it is fully justified by the evidence, though it is, unfortu- 

 nately, not possible to bring before you any adequate digest of 

 that evidence, for it consists of a great mass of tediously minute 

 dental and osteological comparisons of many genera and species. 

 However, some of the more striking parts of the testimony 

 may be made intelligible without an undue amount of detail. 

 The Tylopoda are thus seen to be a very ancient and highly 

 diversified group, comparable in these respects to the Pecora, 

 or true ruminants, which in so many features they closely 

 resemble, though the resemblances have, for the most part, 

 been independently acquired in the two suborders. The Pecora 

 are an Old World group, which underwent a great expansion 

 and diversification in Eurasia, but did not reach this continent 

 till late Miocene times, and they never attained the importance 

 here that they have so long had in the eastern hemisphere. 

 Even at the present time, when they have completely sup- 

 planted and driven out the Tylopoda from North America, they 

 are far less numerous and varied here than in the Old World. 

 Several of their American representatives, such as the bison, 

 sheep, and musk-ox, are very recent immigrants, not occurring 

 in beds older than the Pleistocene. In America the place of 

 the Pecora was taken, to a very great extent, by the Tylopoda, 

 which ran a course of development, in many respects, parallel 



