I/O THE ARCTOG^IC REALM. [CHAP. 



distinct lobes ; and while all the living forms have but three toes 

 to each foot, in some of the extinct hornless species the front limb 

 was four-toed. In the typical genus the number of the front teeth 

 is more or less reduced, but in the extinct Hyracodon and 

 Amynodon of the upper Eocene of North America the full forty- 

 four teeth were developed ; and as allied forms also occur in the 

 Oligocene, it would seem highly probable that the group originated 

 in North America, whence it migrated westwards by way of what 

 is now Bering Strait to attain its most specialised development in 

 the Old World. It is, however, noteworthy that the genus 

 Cadurcotherium, from the French Oligocene, which should ap- 

 parently find a place in this family, and is distinguished by the 

 narrowness of its upper molars, makes a curious approximation 

 in the structure of these teeth to the Neogaeic Homalodontothe- 

 rium 1 . The most specialised representative of the family is the 

 huge Elasmotherium, of the Siberian Plistocene, whose molars shew 

 a resemblance to those of the Equidce. 



Another family of perissodactyles (Titanotheriidce) is typically 

 represented by certain huge, somewhat rhinoceros-like, mammals, 

 generally having a pair of transversely-placed tuberosities on the 

 nasal region of the skull, and characterised by a peculiar arrange- 

 ment of the tubercles on their upper molars. These teeth, which 

 have very short crowns, also differ from those of the rhinoceroses 

 in having the outer wall divided into two lobes by a vertical ridge; 

 while the last lower molar is distinguished from the corresponding 

 tooth of the latter by having a third lobe. The typical genus 

 Titanotherium is mainly North American, where it ranges from the 

 lower Oligocene to the upper Eocene, but is also represented in the 

 Tertiaries of the Balkans, although unknown in those of western 

 Europe. An allied genus (Brachydiastematotherium) has also been 

 recorded from eastern Europe, but all the other members of the 

 family, such as Palceosyops, are North American. This family 

 accordingly appears to have been mainly an American one, but 

 was probably represented in Asia as well as in eastern Europe. 



The remarkable genus Chalicotherium, whose geological range 

 in the Old World extends from the Oligocene of France to the 



1 Supra, p. 82. 



