13 1 8 CLASS MAMMALIA. 



also occurs H. palceindicus, in which the second lower incisor is very- 

 minute, and perhaps disappears in the adult ; while in the existing H. 

 amphibius of Africa, which is found fossil in Europe from the Upper 

 Pliocene of the Val d'Arno to the late Pleistocene, there are only two 

 pairs of incisors in each jaw, the innermost pair in the mandible being 

 of enormous dimensions (fig. 1191). It is evident from the condition 

 prevailing in H. palozindicus that it is the second pair of incisors which 

 is missing in the existing species. Allied to, but smaller than the latter, 

 are H. Pentlandi and H. minutus, whose remains are found in enormous 

 quantities in the caves of Italy and the Mediterranean islands. Finally, 



Mandible of Hippopotamus amphibius. Reduced. 



in the small H. liberiensis, now living in West Africa, the number of 

 lower incisors is reduced to a single pair. The resemblance of the man- 

 dible of Hippopotamus to that of the Anthracotheriidce suggests that 

 both are derived from a common ancestor. 



Family Suid^e. — This family may be taken to include both the 

 existing Dicotylidce and Phacochozridce and the extinct Listriodontidce, 

 since fossil forms indicate a close connection between all these types. 

 The cheek-teeth are typically bunodont and brachydont, the upper 

 true molars carrying four main columns, which may be either simple 

 (fig. 1 195) or of extreme complexity (Hippohyus), but which do not 

 wear into distinct trefoils. Their outer and inner columns may, 

 however, coalesce into transverse ridges (fig. 1193). The skull 

 (fig. 1 192) has a more or less elevated supraoccipital region, from 

 which the profile slopes away to the muzzle, the nasals being fre- 

 quently much elongated; while the mandible has no descending 

 flange at the angle. The canines are frequently large and tusk-like 

 (fig. 1 192), the superior ones being curved upwards, and the lower 

 ones biting against a facet on the outer surface of the upper ones ; 

 but in many of the earlier forms, and in the recent Peccaries, the 



