IV.] ARTIODACTYLES. l6l 



South American Tertiaries ; although, as stated in the last chapter, 

 there are indications of a distant affinity between these and some 

 of the primitive early European perissodactyles. So far as our 

 present knowledge goes, both the perissodactyle and artiodactyle 

 suborders made their appearance in North America during the 

 lower or Wahsatch Eocene ; the former group at least also dating 

 from the same epoch in Europe, where the genus Hyracotherium, 

 which is common to the Bridger and Wahsatch Eocene, and is one 

 of the earliest ancestors of the horses, occurs in the London Clay. 

 Commencing with the Artiodactyla, or even-toed group 

 characterised by the toes corresponding to the third and fourth 

 of the typical pentedactyle limb being symmetrical to a line drawn 

 between them and taking into consideration only such families as 

 have a wide distribution, we have first to do with certain extinct 

 types which serve to connect the pigs with the ruminants. The 

 most pig-like of these are the animals forming the family Chcero- 

 potamidce, characterised by their broad upper molar teeth carrying 

 five blunt tubercles, three of which are on the front half of the 

 crown. Although the typical genus Chosropotamus appears to have 

 been confined to the lower Oligocene of Europe, the much larger 

 animals known as Elotherium (fig. 35) were common to the upper 

 Oligocene of both hemispheres. Nearly allied is the family of the 

 Anthracotheriidce, in which the low tubercles of the molars assume 



FlG. 34. RIGHT UPPER MOLAR OF AncoduS. 



a more or less crescentic, or selenodont structure, thus foreshadow- 

 ing those of the ruminants. Here, again, the typical genus is 

 restricted to the Old World, but the nearly allied Ancodus (Hyo- 

 L II 



