The Artiodactyla have a number of well marked characters, one of the most obvious being the fact that many of the most characteristic forms have large paired outgrowths on the frontal bones. These may be (1) solid deciduous bony antlers, or (2) more or less hollow bony outgrowths which are sheathed with permanently growing horn.
The premolar and molar teeth are usually dissimilar, the premolars being one-lobed and the molars two-lobed; the last lower molar of both the milk and permanent dentitions is almost always three-lobed.
The grinding surfaces of the molar teeth have a tendency to assume one of two forms. In the Pigs and their allies the crowns are bunodont[1], while in the more highly specialised Ruminants the crowns are selenodont[125]. The nasals are not expanded posteriorly, and there is no alisphenoid canal[126]. The thoraco-lumbar vertebrae are always nineteen. The symphysis of the ischia and pubes is very elongated, and the femur has no third trochanter. The limbs never have more than four digits, and are symmetrical about a line drawn between the third and fourth digits; the digits, on the other hand, are never symmetrical in themselves. The astragalus has pulley-like surfaces both proximally and distally, and articulates with the navicular and cuboid by two nearly equal facets. The calcaneum articulates with the lower end of the fibula when that bone is fully developed.
In the Artiodactyla are included the following living groups:—
a. Suina. Pigs and Hippopotami.
b. Tylopoda. Camels and Llamas.
c. Tragulina. Chevrotains.
d. Ruminantia or Pecora. Deer, giraffes, oxen, sheep and antelopes.
Suborder (2). Perissodactyla[127].
In this group there are never any bony outgrowths from the frontals. The grinding teeth form a continuous series, the posterior premolars resembling the molars in complexity, and the last lower molar generally has no third lobe. The cervical vertebrae with the exception of the atlas almost always have markedly opisthocoelous centra, but in Macrauchenia they are flat. The nasals are expanded posteriorly, and an alisphenoid canal is present. The thoraco-lumbar vertebrae are never less than twenty-two in number and are usually twenty-three. The femur has a third trochanter (except in Chalicotherium). The third digit of the manus and pes is symmetrical in itself, and larger than the others, and in some cases the other digits are quite vestigial. The number of the digits of the pes is always odd. The astragalus is abruptly truncated distally, and the facet by which it articulates with the cuboid, is much smaller than that by which it articulates with the navicular. The calcaneum does not articulate with the fibula, except in Macrauchenia. The group includes many extinct forms, and the living families of the Tapirs, Horses and Asses, and Rhinoceroses.