THE UNGULATA, OK HOOFED ANIMALS. 137 



Artiodactyla, or Pair-hoofed Animals. 

 The two main groups of the living Pair-hoofed 

 animals are the Pigs and the Ruminants. We have 

 an approximation to the pigs (above all in the form 

 of the molars) in the hippopotamus, which con- 

 stitutes a side group. The characteristic feature, 

 which is met with even in the earliest known 

 forms, is the peculiar formation of the crowns of 

 the molar teeth; and we shall accordingly desig- 

 nate the Pigs as tuberculate-toothed animals or 

 Bunodonta ; the Ruminants as crescentic-toothed 

 animals or Selenodonta. In animals of the pig 

 species the enamel of the surface of the crowns 

 shows elevations. The almost four-cornered crown 

 (Fig. 15, to the left) shows a front and a back wall 

 (v, h), an outer and inner side, an outer and an 

 inner front tubercle (AI), and an outer and an 

 inner back tubercle (ai). The tooth varies in 

 accordance with this scheme. Elevations of the 

 enamel are met with also in the ruminant type, 

 but present the form of a crescent (Fig. 5, on the 

 right), and much deeper folds. 



1. THE SUIDJE, OK PIGS. 



The Pigs are represented in Europe by the 

 widely distributed genus Sus, and a few others of 



