UNGULATA. 311 



by the formation of its lower extremity, which carries two 

 pulley-like surfaces, with which the bones of the two 

 functional toes (the third and fourth) articulate. 



"Since all the Ungulates displaying this second modi- 

 fication of foot-structure agree in having the third and 

 fourth toes arranged symmetrically to a line drawn 

 between them, they are collectively termed the Even-Toed 

 Ungulates. 



"It is accordingly evident that although a few living 

 Ungulates, like the elephant and the hyrax,' retain a gen- 

 eralized type of foot, the greatest number of the living 

 representatives of the order are characterized by their 

 more or less markedly specialized feet. 



"As regards their teeth, the Ungulates are character- 

 ized by those of the cheek-series having broad crowns, 

 surmounted either by columns or transverse ridges, and 

 adapted for grinding and masticating vegetable sub- 

 stances. In the more specialized forms, like cattle and 

 horses, these cheek-teeth have their columns or ridges of 

 great height and closely approximated to one another, in 

 consequence of which the bases of the hollows or valleys 

 by which these columns or ridges are separated from one 

 another, cannot be seen when the tooth is unworn, while 

 the pattern produced on the crown by the wearing down 

 of these columns or ridges is complex. On the other hand, 

 in the more primitive types, such as pigs and tapirs, the 

 crowns of the cheek-teeth have low columns, or ridges, 

 so that the bases of the intervening valleys can be dis- 

 tinctly seen at all stages of wear. 



"It should also be mentioned, that whereas in Carni- 

 vores the upper molar teeth are generally of the primi- 

 tive triangular type, in all existing Ungulates they have 

 assumed the quadrangular form. The food of the Ungu- 

 lates consisting in most cases entirely of vegetable sub- 

 stances requiring much mastication, is the inducing cause 

 for the complex structure of the cheek-teeth in the more 

 specialized kinds ; and to the same cause may be at- 

 tributed the circumstance that Ungulates always retain 

 the full number of molar teeth, and, except in the camels, 

 at least three out of the typical four premolars. 



