_Hyotherium_ (which is regarded as identical with _Palaeochoerus_) has a sharp sagittal crest; the orbit is nearly but not quite closed. The canines are not strongly developed. The upper canines have double fangs as in _Triconodon_ among extinct mammals, and as in the Hedgehog and other forms among living Mammalia. The premolars have the cutting and serrated edge of those of some other Pigs, a feature which gives them a curious resemblance to the "grinding" teeth of Seals. The molars are tuberculate, and like those of living Pigs. It is European and Indian in range, and Miocene. The genus _Choeropotamus_ has a complete dental formula save for the loss of a premolar in the lower jaw. Though it has lost this tooth, it is from an older stratum than some of those forms which have retained that premolar; it has been found in the Upper Eocene of the Isle of Wight and of the neighbourhood of Paris. The American and Miocene _Chaenohyus_ has lost the corresponding teeth of the upper jaw. _Homacodon_[190] is a genus consisting of several species, which has a bunodont and complete dentition. The molars are sextubercular in the upper jaw. _H. vagans_ was of about the size of a Rabbit, and it appears to have had a curved neck. The limbs had five digits, as is so generally the case with Eocene Ungulates. It is known from the Middle Eocene of Wyoming. 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 {281} 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. [Illustration] 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. A. TRAGULINA.