PEEISSODACTYLA. 319 



into the single order of the Ungulata, or Hoofed Quadrupeds, 

 •and the following are the characters of the order : — 



All the four limbs are ijresent, and that portion of the toe 

 ivhich touches the ground is ahvays encased in a greatly-ex- 

 panded nail, constituting a " hoof. " There are rarely more 

 than four full-sized toes to each limb. Oioing to the encase- 

 ment of the toes in hoofs, the limbs are useless for 'prehension, 

 and only subserve locomotion ; hence clavicles are always ivant- 

 ing in the entire order. There are always tivo sets of enamelled 

 teeth, so that the animal is diphyodont. The molar teeth are 

 massive and have broad croivns, adapted for grinding vegctahle 

 substances. 



The order Ungidata is divided into two primary sections : 

 the Perissodactyla, in wliich the toes or hoofs are odd in 

 number (one or three, or rarely five), and the Artiodactyla , 

 in which the toes are even in numlier (two or four). 



Both these great sections were differentiated as early as 

 the Eocene Tertiary, in which formation are found the oldest 

 remains of Ungulates which have been as yet discovered. 

 The ancestral types from which these sections were derived 

 are still unknown to us, but it is to be expected that these 

 will be found to possess the full five digits wliich are nor- 

 mally present in the mammalian limb. It might, therefore, 

 be not unadvisable to create a third section of the Ungidata, 

 wdiich might be termed Teleodactyla, for the reception of 

 those forms in which the foot is five-toed. In the mean- 

 while, however, the few known pentadactylous Ungulates 

 (the Coryphodontia) may be temporarily retained in the 

 Perissodactyle section of the order, with which they have 

 affinities in other respects. 



Section A. Pepjssodactyla. 



The section of the Perissodactyle Ungulates includes the 

 living types of the Ehinoceroses, Tapirs, and Horses, to- 

 gether with a vast number of extinct forms, only the more 

 important of which can be alluded to here. The characters 

 of the section are as follows : — 



The hind-feet are odd-toed in cdl (fig. 624, b), and the fore- 



