174 



HIGH SCHOOL ZOOLOGY. 



during tlie Tertiary periotl until they attained the statui'e of the 

 horses of the present day. 



29. The great bulk of the Ungulates belong to the order 

 Artiodactyla, in wliich the third and fourth toes equally 

 support the weight of the body. Here we have also primitive 

 forms, and specialised forms adapted for rapid locomotion ; the 

 primitive forms being, as in the last group, swamp loving or 

 aquatic creatures, with comparatively hairless skin, and with 

 teeth much more nearly approaching the tuberculate type than 

 do those of the more specialised forms. As far as the dentition 

 is concerned, the swine are the most primitive of the living 

 species, but the Hippopotiimus is supported by all four toes, and 

 therefore, in this respect, it is the more primitive form. 



ri;;. 115.— Reduction of the lateral toes, and coalescence of metapodials into a can- 

 non-lx)ne in Artiodactyla. (After Gaudry.) 



A, pi^'; B, Hycemuschus ; C, roe; D, antelope {Calotragug) ; E, sheep; F, 

 embrj'O calL 



These constitute two families, the Hipjiopotamidce and the Suidce, 

 which differ from the remaiuing Artiodactyla in not chewing the cud. 

 The first family has only a single genus, which, at the premie day, is 



