The order of the Ungulata, or Hoofed Quadrupeds, is one of the largest and most important of all the divisions of the Mammalia. It comprises three entire old orders - namely, the Pachydermata, Solidungula, and Ruminantia.
The first of these old divisions - that of the Pachydermata - included the Elephants, Rhinoceros, Hippopotamus, Tapirs, and the Pigs, all characterised, as the name implies, by their thick integuments. The name is still used to express this fact, though the order is now abandoned, and is merged with that of the Ungulata; the Elephants alone being removed to a separate order under the name of Proboscidea.
The second old order - that of the Solidungula or Solipedes - included the Horse, Zebra, and Ass, all characterised by the fact that the foot terminates in a single toe, encased in an expanded hoof. The name Solidungula is still retained for these animals, as a section of the Ungulata.
The third old order - that of the Ruminantia - includes all those animals, such as Oxen, Sheep, Goats, Camels, Giraffes, Deer, and others, which chew the cud or "ruminate," and have two functional toes to each foot, encased in hoofs. The name Ruminantia is still retained for these animals, as constituting a most natural group of the Ungulata.
All these various animals, then, are now grouped together into the single order of the Ungulate, or Hoofed Quadrupeds, and the following are the characters of the order:
All the four limbs are present, and that portion of the toe which touches the ground is always encased in a greatly-expanded nail, constituting a "hoof" Only in a few extinct forms (the Cory-phodontidae) are there more than four full-sized toes to each limb. Owing to the encasement of the toes in hoofs, the limbs are useless for prehension, and only subserve locomotion; hence clavicles are always wanting in the entire order. There are always two sets of enamelled teeth, so that the animal is diphyodont. The molar teeth are massive and have broad crowns, adapted for grinding vegetable substances.
In accordance with the number of the digits (fig. 388), the order Ungulata is divided into two primary sections: The Perissodactyla, in which the toes or hoofs are odd in number (one or three, or, in the extinct Coryphodontidae, five), and the Artiodactyla, in which the toes are even in number (two or four).
Fig. 388. - Feet of Ungulata. A, Fore-foot of Tapir (Tapirus Malay anus); B, Perisso-dactyle fore-foot of Rhinoceros Sumatrensis; C, Artiodactyle foot of Pig (Sus scrofa). The figures indicate which of the normal five digits are present in each foot. (After Flower.)