THE hoofed quadrupeds are so called because they possess hoofs, from which fact the order Ungulata takes its name,[260] and they include animals of widely different appearance, such as the Horse, Rhinoceros, Giraffe, Camel, and the like. They are classified into two sub-orders, according to the odd or even number of toes, those having an odd number on the hind foot being termed the Perissodactyla,[261] such as the Horse, Tapir, and Rhinoceros; and the Artiodactyla,[262] or animals with an even number of toes on their hind feet, such as the Pig, Hippopotamus, Sheep, Ox, Deer, and the like. All the animals belonging to the order feed upon vegetables, with the exception of the Pig and Peccary, which are omnivorous; and none of them are provided with sharp-edged cutting back teeth, adapted for dividing flesh, such as are found in the Carnivora—Lions, Tigers, Wolves, and Hyænas. The odd-toed Ungulates come first.
SUB-ORDER PERISSODACTYLA.
The odd-toed animals consist of three living families—(1) The Equidæ, or Horses; (2) the Tapiridæ, or Tapirs; (3) the Rhinocerotidæ, or Rhinoceroses; and two extinct families—(1) the Palæotheridæ, or Palæotheres (παλαιός, old; θηρίον, beast); and (2) the Macraucheniadæ (μακρός, long; αὐχήν, neck). In all the animals belonging to the group, the number of dorso-lumbar vertebræ is not fewer than twenty-two, the third or middle digit of each foot is symmetrical, the femur or thigh-bone has a third trochanter, or knob of bone on the outer side, and the two facets on the front of the astragalus or ankle-bone are very unequal. When the head is provided with horns, they are skin-deep only, without a core of bone, and they are always placed in the middle line of the skull, as in the Rhinoceros.
In the Perissodactyla the number of toes is reduced to a minimum. Supposing, for example, we compare the foot of a Horse with one of our own hands, we shall see that those parts which correspond with the thumb and little finger are altogether absent, while that which corresponds with the middle finger is largely developed, and with its hoof, the equivalent of our nail, constitutes the whole foot. The small splint bones, however, resting behind the principal bone of the foot represent those portions (metacarpals) of the second and fourth digits which extend from the wrist to the fingers properly so called, and are to be viewed as traces of a foot composed of three toes in an ancestral form of the Horse, which we shall discuss presently. In the Tapir, the hind foot is composed of three well-developed toes, corresponding to the three middle toes in man, and in the Rhinoceros both feet are provided with three toes formed of the same three digits. In the extinct Palæotherium also, the foot is constituted very much as in the Rhinoceros.
FAMILY I.—EQUIDÆ, THE HORSE-TRIBE.
TARPAN.
The Equidæ, or Horse-tribe, comprise several living and many extinct species. Three living members are restricted, in a state of nature, to Asia and Africa, and are divided into the true Horses, which have horny patches or callosities on the inner sides of both pairs of limbs—above the wrist in the fore, and on the inner side of the metatarsus on the hind limbs—and the Asses, which possess such callosities only on the fore-limb. With the latter are classed the Zebras and the Quaggas. All the existing and some of the extinct members of the family, are characterised by the feet being formed of one perfectly developed digit or toe only, the others being present in a rudimentary shape as the splint-bones. In the extinct Hipparion, however, and Anchithere, as we shall see presently, the accessory toes are well developed.
WILD HORSE OF TARTARY.