AMERICAN RUMINANT-LIKE MAMMALS. 247 



tion is that the group originated in North America, and after 

 sending out migrants to the other regions, became extinct 

 here. 



The study of the peculiarly American selenodonts can most 

 conveniently begin with those of White River times, because 

 they are the best known, nearly every genus being represented 

 in the collections by complete skeletons, and because they were 

 then perhaps in the most characteristic stage of their develop- 

 ment. As to the number of families represented in the forma- 

 tion, it is difficult to reach a conclusion, and almost as good 

 reasons may be given for grouping them into seven families as 

 into four, the arrangement here adopted. The position of only 

 one of these groups has long been understood and very gener- 

 ally agreed upon, and this is the family represented in White 

 River times by Poebrotheriimi, a genus which, there can be 

 little doubt, is the ancestor of the modern Tylopoda, or, at the 

 very least, of the llamas. The whole appearance of the skele- 

 ton, with its small tapering head, long neck, and elongate 

 slender limbs and feet, is like that of a small llama, but of 

 course it is very much more primitive and less specialized than 

 the existing members of the group. The teeth are still undi- 

 minished in number; the canines are hardly larger than the 

 incisors and only beginnings of diastemata are visible ; the pre- 

 molars are much elongated in the antero-posterior direction, and 

 the molars are commencing to take on the prismatic or hypso- 

 dont shape. The skull is unmistakably tylopodan in character, 

 with its triangular form and slender tapering muzzle, and while 

 primitive features are retained, the enlarged tympanic bulla 

 filled with cancellous bone has already been formed. The 

 cervical vertebrae display the tylopodan peculiarity of a con- 

 cealed vertebrarterial canal, perforating the neural arch, and 

 only in the sixth vertebra does the canal occupy its normal 

 position. The limbs and feet are already very elongate ; the 

 ulna and radius are coossified, and the fibula is completely 

 reduced ; the feet are didactyl, the lateral digits being reduced 

 to mere nodules. The phalanges are slender and the unguals 

 long and pointed, the shape of the latter showing that the 

 hoofs were like those of the deer and antelope, and that the 



