IV.] ARTIODACTYLES. 163 



potamus] has the same approximate distribution as Elotherium, 

 although it also occurs in the Miocene of northern India, together 

 with Anthracotherium and a genus (Tetraconodon] closely allied to 

 Elotherium. 



A group of smaller Eocene and Oligocene mammals, con- 

 stituting the family Ccenotheriidce, differ from the preceding in 

 that their upper molars have two cusps on the front, and three on 

 the hinder half of the crown. Here none of the genera are com- 

 mon to the two sides of the Atlantic, Ccenotherium and Dichobumis 

 being European, while the latter is represented in the Bridger 

 Eocene by the closely allied Homacodon. It has been suggested 

 that this group includes the ancestors of the camel tribe. 



The latter group (Camelidce), now represented only by the 

 camels (Camelus) in the Old World, and the guanacos, vicunas, 

 and their domesticated allies the llamas in South America, was 

 formerly widely distributed in Arctogaea, which was doubtless its 

 original home, since, as we have seen in the preceding chapter, 

 the llamas and their allies are but comparatively recent immigrants 

 into Neogaea. Camelus itself is represented in a fossil state in the 

 Pliocene of northern India and the Plistocene of Algeria ; but no 

 other Old World representatives of the family are known. Very 

 different, however, is the case with North America, where, from 

 the Plistocene downwards, we meet with a host of extinct types, 

 such as Pliauchenia, Procamelus, Protolabis, etc., gradually con- 

 necting the existing forms with a small animal from the middle 

 Oligocene known as Poebrotherium, which exhibits many very 

 generalised characters. A still earlier representative of the family 

 in Leptotragulus, of the lower or Uinta Oligocene, which itself may 

 be sprung from the aforesaid Homacodon of the underlying Bridger 

 beds. It is thus perfectly evident that the cameloids were ori- 

 ginally a N. American group. One branch crossed the area now 

 occupied by Bering Strait to found the camels of the Old World ; 

 while, probably at a later date, a second branch passed over the 

 isthmus of Panama to persist in the guanacos, vicunas, and 

 llamas of South America. The disappearance of the whole group 

 from the northern half of the New World is a very remarkable fact, 

 but is paralleled by that of the elephants, rhinoceroses, lemurs, and 

 several other groups. 



II 2 



