Both the existing species are now exclusively confined to Africa, but in the Pleistocene and Pliocene periods the genus was widely spread over the Old World. Thus in the Upper Pliocene of the Continent and the Pleistocene of England we meet with remains of a very large fossil Hippopotamus which cannot be specifically distinguished from _H. amphibius_. In the Pleistocene and Pliocene of India there are two species having three pairs of incisors in both jaws. Of these _H. palæindicus_ has the second pair in the lower jaw very minute, and evidently just about to disappear; from which we learn that it is this pair which is missing in _H. amphibius_. In the still more generalised _H. sivalensis_ the three incisors in the lower jaw are of equal size. Hexaprotodont species also occur in the Upper Tertiaries of Burma and Algeria. Small tetraprotodont species (_H. pentlandi_ and _H. minutus_) have left their remains in enormous quantities in the caves and fissures of Sicily and Malta. _Family_ SUIDÆ. An elongated mobile snout, with an expanded, truncated, nearly naked, flat, oval terminal surface in which the nostrils are placed. Feet narrow; four completely developed toes on each. Hoofs of the two middle toes with their contiguous surfaces flattened. The outer (second and fifth) digits of existing forms not reaching to the ground in the ordinary walking position. Teeth variable in number, owing to the suppression in some forms of an upper incisor and one or more premolars. Incisors rooted. Upper canines curving more or less outwards or upwards. Stomach simple, except for a more or less developed pouch near the cardiac orifice. A cæcum. Colon spirally coiled. Confined to the Old World. The mandible has no descending flange at the angle. The crowns of the molars do not wear into such distinct trefoils as in the Hippopotamus, and are oblong in shape. The last molar of both the upper and lower jaw (Fig. 102) has an additional hinder lobe or talon, varying in size in the different species. The upper premolars are simpler than the true molars. [Illustration: FIG. 102.—Grinding surface of a worn third right lower molar of the Wild Boar (_Sus scrofa_). After Owen.]