ARTIODACTYLA. To some of our domestic animals the term "cloven-hoofed" is commonly applied. These are the cow, the sheep, the goat and the pig. Deer are also "cloven-hoofed" animals, as are giraffes, camels, antelopes and certain other ani mals less well-known to us. Scientifically speaking, however, "cloven-hoofed" is not a good term ; it suggests that these animals have, like a horse, a single hoof, and that this hoof is cleft down the middle. If the foot and its skeleton are examined more care fully it will be seen that each half of the so-called "cloven hoof" is really a little hoof in itself, flattened on the side which faces its fellow and belonging to a different toe. These two toes are equal in size and carry the weight of the animal, which always stands, as it were, on tip-toe, with ankle and wrist raised high off the ground, the terminal hoof-clad joint of the toe alone resting on the latter. Some "cloven-hoofed" animals have only these two toes, but some, such as the pig, have another pair of similar toes out side the central pair, while the hippopotamus has four toes of al most equal breadth, all but one of the ancestral five. In the pig each of the small lateral toes has its full complement of toe bones and a perfect little hoof at the end, but they are much shorter than the central toes and so only come into use when the animal is walk ing on soft swampy ground—then they help to prevent it from sinking in too far. In the cow and sheep vestiges only of these lat eral toes are left, the so-called "dew claws" ; in a giraffe they have quite disappeared. Inter mediate conditions are found in a peccary, in a chevrotain, and in deer (see fig. I) . All these forms agree, however, in having the axis of symmetry of the foot passing between the third and fourth toes of the ancestral five, these two forming an equal and symmetrical pair on either side of it. The hind foot fundamen tally resembles the forefoot in construction, but the reduction of the lateral toes in the former has generally gone further, since the hind foot is of greater im portance in propelling a running animal, while the forefoot is mainly concerned with supporting its weight. Fig. 2 shows the ar rangement of the bones in a pig's hind foot. Of seven small ankle or tarsal bones the two upper ones form a joint, the "hock joint," with the long bones of the shin, while the lower ones sup port a row of four metatarsals, one to each toe. The metatarsals in their turn each support three phalanges, the lowest of which are flattened and pointed and covered by the horny hoofs. The inner of the two bones at the ankle joint, the astragalus, is of a very characteristic shape, since it not only has a pulley-shaped surface for articulation with the shin bone or tibia, as in all mammals, but also a similarly shaped surface at the lower end, almost equally divided between the two small tarsal bones beyond it, the navicular and cuboid. An astragalus of this peculiar shape is found in nearly all cloven-hoofed (artiodactyl) mammals, and appears to be a consequence of the tendency to make the two enlarged central toes do all the work of the foot. The outer bone of the shin (fibula) is very reduced, so that nearly all the animal's weight is transmitted through the tibia to the astragalus; half the weight then passes through the cuboid to the outer of the central toes, half through the navicular to the inner toe by way of the iform. Stresses are transmitted from the ground upwards by the same path. In the more advanced cloven-hoofed mammals, such as the deer and sheep, the lying plan of the hind foot mains the same, but the ankle is strengthened by fusion of ular and cuboid, and the central metatarsals are joined to form a "cannon-bone" whose upper end is broadened out to support the whole tarsus. Fossilized astragali of the double-pulley shape have been found in strata of Lower Eocene age associated with teeth of the "tritubercular" pattern common to the primitive bers of nearly all mammalian orders. The artiodactyl type of foot symmetry is therefore clearly of very ancient origin. It may well have been evolved dependently in more than one group of primitive mammals, but so far no astragalus has been found intermediate in shape be tween the double pulley and the type with a rounded lower end which appears to have been that of the original mammalian stock.
Classification.—The "cloven-hoofed" mammals were first grouped together as an independent order by Richard Owen in 1847, mainly on account of the structure of their feet. He termed them Artiodactyla (Gr. apTCOS, even, and (5aervXos, a finger or toe) to distinguish them from those other hoofed mammals, the horses, tapirs and rhinoceroses (PERISSODACTYLA, q.v.) in which a single central toe does the main work of each foot, the other toes being smaller and grouped symmetrically on either side of this central one, or reduced to mere vestiges. A conservative classifica tion of the living Artiodactyla is as follows : The Ruminantia are a well defined group united by characters of the skull, dentition and stomach, as well as by the structure of their feet. The Suina, on the other hand, except for their foot structure, have almost no important characters in common with the Ruminantia, nor are the Suidae very closely related to the Hippopotamidae. Indeed, before Owen established the order Artio dactyla the Suina were kept apart from the Ruminantia, which constituted an order in themselves. The Suina shared Cuvier's old order Pachydermata ("thick-skinned" animals) with the rhinoc eroses, tapirs and elephants, on no better grounds than that all were thick-skinned, massively built, hoofed mammals, with a number of toes to each foot and of an omnivorous diet. Owen was not a believer in evolution, at any rate in the Darwinian sense, but his rearrangement of the Pachydermata fitted in well with the new way in which people began to look at the problems of classifica tion when once the idea of evolution had taken root in their minds.
That one animal had four toes and another only two now seemed no longer a sound reason for putting them in different orders, for it was realised that every two-toed animal must once have passed through a four-toed stage and it appeared of much greater impor tance that in one group of animals there was a tendency to make one central toe do all the work of the foot, while in another group the tendency was to utilize two equally developed central toes. From 1850 onwards discoveries of fossil mammals became more and more numerous. Among these were many new types of Artio dactyla which would not fit into the old subdivisions of that order, some being intermediate while others were entirely novel. The parts of fossil animals most com monly preserved in good condi tion are the teeth, owing to their hard protective coat of enamel, and so it came about that the many new attempts to subdivide the order were largely based on characters of the teeth alone, especially of the molar teeth.
Another classification, that of Dr. H. G. Stehlin, is based on the number and position of the molar cusps (see Abel's Die vorzeit lichen Sdugethiere).
Each of these classifications produces an entirely different grouping of the Artiodactyla, indicating that the use of molar characters alone is not sufficient to express the inter-relationships of living and extinct animals. The grouping into Suina and Ruminantia given in the present article follows the old division of the living families. In allotting the extinct families to one or the other group attention has been paid to the structure of the skull as well as to the characters of the dentition. The name Ruminantia, suggesting a capacity to ruminate ("chew the cud"), suffers from the defect that we know nothing about the digestive processes of the extinct forms.