POSITION AND STRUCTURE OF HORSE 9 



it is invariably 26. If we exclude from this enu- 

 meration the seven cervical vertebrae common 

 to all ungulates, this may be expressed in another 

 way by saying that whereas in the odd-toed group 

 the number of trunk- vertebrae may be 22 or 23, in 

 the even-toed group it is invariably 19. 



Other differences in the skeleton, as well as 

 certain peculiarities in the teeth of the two groups, 

 need not be mentioned here ; but it may be 

 observed that no perissodactyle has the complex 

 type of stomach characteristic of the ruminating 

 artiodactyles. 



Although, as already mentioned, the Perisso- 

 dactyla are represented at the present day only by 

 three families — the EqtiidcF^ Rhinocerotidce, and 

 TapiridcB — the living members of each of which 

 may be included in a single genus, during the 

 Tertiary period they comprised several extinct 

 families and a large number of genera. Nor is 

 this all, for whereas, with the exception of the 

 tapirs, which are common to Malaya and Tropical 

 America, the group is nowadays restricted to the 

 Old World, in past times it was abundantly repre- 

 sented in the New World, where the three existing 

 families (together with certain extinct ones) occurred 

 in North America, while the Equidce succeeded in 

 effecting an entrance during the latter part of the 

 Tertiary period into South America. 



Even this, however, does not represent the real 



