Contrasted with these, _Artiodactyla_ have always an even number of functional digits, the third and fourth reaching the ground symmetrically, bearing the weight and forming the "split hoof;" the second and fifth remain, in most cases, as mere vestiges, showing externally as the accessory hoofs or dewclaws; in the hippopotamus alone they are fully developed and the animal has a four-toed foot. In deer and bovine animals the incisors and frequently the canines have disappeared from the upper jaw, and the molars are unlike the premolars in having two lobes instead of one. The stomach is always more or less complex; at its extreme reaching the ruminant type with four compartments, in association with which is a caecum reduced in size and simple in form. Nearly all have horns or antlers, at least in one sex. Most split-hoofed animals are ruminants, but there is a small remnant, probably of early types, which are not. The present ungulates may be summed up in this way: Odd-toed: _(Perissodactyla)_-- Horse, Ass, Rhinoceros, Tapir. Even-toed: _(Artiodactyla)_-- Non-ruminants-- Hippopotamus, Swine, Peccaries. Ruminants-- Camels, Llamas, Chevrotains, Giraffe, Antelopes, Sheep, Goats, Musk-ox, Oxen, Deer. The non-ruminant artiodactyls need not detain us long. Hippopotamuses are little more than large pigs with four toes; they were never American, though many species, some very small, are found in the European Tertiary. The two existing species are African. In the western hemisphere swine are represented by the peccaries, differing from them chiefly in having six less teeth, one less accessory toe on the hind foot, and in a stomach of more complex character. Peccaries also have the metapodial bones supporting the two functional digits fused together at their upper ends, forming an imperfect "cannon bone," which is a characteristic of practically all the ruminants, but of no other hoofed beasts. One species only enters the United States along the Mexican border. All non-ruminant ungulates have from four to six incisors in the upper jaw; the canines are present, and sometimes, as in the wart hogs, reach an extraordinary size.