Artiodactyla. The great mammalian order Ungulata (see MAMMALIA, UNGULATA) is divided into two groups; first, the Perissodactyla, including the horse, tapir, and rhinoceros, besides a multitude of extinct forms, and distinguished by the third digit of each limb being symmetrical in itself, by the presence of an odd number of digits on the hind-foot, by the number of dorso-lumbar vertebrae being at least twenty-two, and so on; while the second sub-order, the Artiodactyla, possess the third digit, unsymmetrical in itself, but forming a symmetrical pair with the fourth digit. While the hind-foot bears an even number of digits, the number of dorso-lumbar vertebrae never reaches twenty-two, and rarely exceeds nineteen. Numerous minor osteological differences exist between the two sub-orders, which broadly correspond to the ancient divisions of solid-hoofed and cloven-hoofed respectively. In the article FOOT will be found some details as to the differences in the feet of various animals of the ungulate order.
The Artiodactyla again divide into two groups —the Non-Ruminantia and the Ruminantia. The former have usually more than one pair of upper incisors, and the molars have a more or less tuberculated pattern, whence they are frequently termed Bunodonta. The metacarpal and metatarsal bones remain separate, and there are no horns. The stomach has rarely more than two divisions. The Non-Ruminantia include two existing families, Suidæ and Hippopotamidæ. The Suidæ (pigs) have the skin moderately thick and hairy; the third and fourth toes are much longer than the second and fifth. The teeth are frequently as many as forty-four, and the molars are multituberculate. The Hippopotamidæ have the skin extremely thick, with scanty hairs; the head, body, and limbs extremely massive; and the four toes all resting on the ground. The Ruminantia have never more than one pair of upper incisors. In the lower jaw, the canines closely resemble and are situated beside the six incisors, which thus seem to have increased to eight. The molars bear a double series of crescentic ridges, whence the name Selenodonta is frequently applied to the ruminant group. The stomach has at fewest three, and usually four divisions. Thus in the sheep or ox, the cardiac portion of the stomach is differentiated into the enormous rumen or paunch, and the reticulum or honeycomb stomach, with which it communicates. After the fodder has been chewed again, it passes readily into the third division, the psalticrum, or manyplies, which acts as a filter, and only allows the finely comminuted portions of the food to enter the highly glandular abomasum or rennet stomach, in which gastric juice is secreted, and proteid digestion goes on.
The existing groups of ruminants are the Tragulidæ, the Cotylophora, and the Camelidæ. The Tragulidæ (sometimes erroneously termed musk-deer) are the least differentiated forms, and show interesting affinities to the non-ruminants. The Cotylophora, including the ox and deer tribes (Bovidæ and Cervidæ), are the central family, which are broadly distinguished by the cotyledonary placenta, and the structure of their horns; those of the Cervidæ being naked, deciduous, and annually renewed processes of the frontal bones; while those of most Bovidæ (sheep, antelopes, oxen, buffaloes) are non-deciduous processes of the frontal bones, covered by the thickened and hardened epidermis known as horn. But in one sub-family, the giraffes, the horns arise as separate ossifications, and are covered by hairy skin.
The third family, the Camelidæ, are aberrant ruminants. They walk on broad integumentary cushions, developed below the phalanges of the third and fourth toes, which are alone developed, the nails not forming hoofs. Large pointed canines are present in each jaw. The stomach has a characteristic structure. The placenta is diffuse. There are only two existing groups—the Camels of the Old World, and the Llamas of the New.
See RUMINANTIA, BOVIDÆ, DEER; also PIG, HIPPOPOTAMUS; ALPACA, ANTELOPE, BUFFALO, CAMEL, GIRAFFE, LLAMA, OX, SHEEP, &c.