POSITION AND STRUCTURE OF 



its immediate living relatives are broadly distin- 

 guished from all other modern mammals by the 

 reduction of the number of toes on each foot to 

 one. During the later part of the Tertiary, or 

 latest, period of geological history there existed, 

 however, a number of animals agreeing in all 

 essential characters with the modern horse and its 

 relatives, but with three toes to each foot, although 

 the lateral ones were so small as to be of no 

 functional importance. These three-toed horses, 

 of which there is more than one generic type, had 

 tall-crowned cheek-teeth differing from those of 

 the modern horse only in certain comparatively un- 

 important details of structure, and in the somewhat 

 inferior height of their crowns. Somewhat earlier 

 in the Tertiary are found remains of other three- 

 toed horse-like animals, which differ much more 

 markedly from the modern type. Among their 

 more salient differences are the relatively larger size 

 of the lateral toes, which, in some cases at any 

 rate, were at least partially of functional use ; and, 

 more important still, the quite short crowns of their 

 cheek-teeth. 



Now in defining the Equida it is found con- 

 venient to take tallness of crown in the cheek- 

 teeth as the main distinctive character, and to 

 include in that family only those species in which 

 this feature is distinctly developed. The short- 

 crowned species are accordingly referred to separate 



