ORDER UNGULATA. 



1319 



development of these teeth is much less marked. The digits are 

 usually four in number, but in the existing forms (fig. 1185) only 



Fig. 1 192. — Right lateral aspect of the skull of the Wild-boar (Szts scrofd). Reduced. 

 (After Gray.) 



the two middle ones touch the ground. The dental formula is very 

 generally the typical one. 



The extinct genus Listriodon differs from all the other members 

 of the family in that the true molars (fig. 1193) carry a pair of 

 simple transverse ridges. The dental formula is 



/ 



M. 



the canines form large 



Fig. 1 193. — The sec- 

 ond left upper true mo- 

 lar of Listriodon splen- 

 dens; from the Middle 

 Miocene of France. 



I C. \ Pm. 3 



3 1 3 3 



tusks ; the last upper premolar is simpler than 

 the true molars ; and the anterior premolars are 

 relatively wide. The skull is essentially that of 

 a Pig. Remains of Listriodon occur in the 

 Middle Miocene of the Continent (where they 

 have been described under the names of Lophio- 

 chozrus and Tapirotheriuni), and also in the Plio- 

 cene Siwaliks of the Punjab and Sind. The 

 molars present the same relation to those of Sus 

 as is borne by the molars of Dinotherium to those of many species 

 of Mastodon. 



Leaving this aberrant type, we may turn to the typical genus Sus, 



in which the normal dental formula is I. -, C. -, Pm. -, M. -, 



3 1 4 3 



although the first premolar is absent in some fossil species, and also 

 in the African Potamochozrus, which cannot be palaeontologically 

 separated. The canines are developed into tusks (fig. 1192), 

 although they are small in the earlier species. The crowns of 

 the upper true molars are oblong, and both the upper and lower 

 last true molars (fig. 1194) have a third lobe, although its degree 

 vol. 11. 2 E 



