Remains of very generalised swine-like animals have been abundantly found in Tertiary formations both in America and Europe. In the former continent they never (so far as present evidence indicates) underwent any great diversity of modification, but gradually dwindled away and almost died out, being only represented in the actual fauna by the two closely allied species of Peccary, among the smallest and most insignificant members of the group, which have existed almost unchanged since the Miocene age at least, if the evidence of teeth alone can be trusted. In the Old World, on the other hand, the swine have played a more important part in recent times, having become widely distributed, and throwing off some curiously specialised forms. At the present time, though not very numerous in species, they range through the greater part of the Old World, except within or near the Arctic Circle, although, in common with all the other members of the great Ungulate order, they were completely absent from the whole of the Australian region, until introduced by man in very recent times. The existing swine-like animals may be divided naturally into three families:—I. _Hippopotamidæ_; II. _Suidæ_, or true Pigs; III. _Dicotylidæ_, or Peccaries.[176] _Family_ HIPPOPOTAMIDÆ. [Illustration: FIG. 100.—Grinding surface of a worn molar of _Hippopotamus amphibius_. (From Owen.)] Muzzle very broad and rounded. Feet short and broad, having four subequal toes, with short rounded hoofs, all reaching the ground in walking. Incisors not rooted, but continuously growing; those of the upper jaw curved and directed downwards; those of the lower straight and procumbent. Canines very large, curved, continuously growing; those of the upper jaw directed downwards. Stomach complex. No cæcum.