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.