Group II.— Ruminantia

The Selenodontia or Ruminantia form the second division of existing Artiodactyles. The characters of the teeth, which give them their name, have already been referred to. They also differ in that there are never more than a single pair of incisors in the upper jaw, and very usually there are none. As a general rule the third and fourth metacarpals and metatarsals become united to form a cannon bone. To this there is but one exception, the African Hyomoschus. Moreover, the second and fifth digits are nearly always rudimentary, and may practically disappear altogether. Here again the Tragulidae are an exception. The Ruminantia are so-called on account of the fact that they "ruminate," that is, after the food has been rapidly swallowed, it is forced back up the gullet and more thoroughly masticated. Associated with this is a complex stomach, which is divided into several compartments. This stomach has at least three compartments, as in the Tragulidae; but it has usually four. Its characters are illustrated in Fig. 146. The majority of the Selenodontia possess horns, which are partly formed of solid protuberances of the frontal bones. In the Giraffe they are somewhat different.

Fig. 146.—Stomach of Ruminant opened to show the internal structure. a, Oesophagus; b, rumen; c, reticulum; d, psalterium; e, abomasum; f, duodenum. (After Flower and Lydekker.)

This group may be divided into—A. Tragulina, Chevrotains; B. Tylopoda, Camels, Lamas; and C. Pecora, Deer, Antelopes, Oxen, Giraffes, Goats, Sheep.