The carpal bones of the Artiodactyla alternate in their articulation; the primitive state of affairs[[172]] is not retained even in the earliest types. The femur has no third trochanter, so prevalent in the Perissodactyles. In the hind-foot the calcaneum has an articular facet for the fibula, which is not characteristic of the Perissodactyla. In the more modern forms, e.g. the Cervidae, the navicular and cuboid become fused into one bone; and there are even further fusions which will be referred to later as characteristic features of different groups. It is interesting to notice that the reduction begins earlier and is clearer in the hind-foot than in the fore. One
can see how this may be purely adaptive, the push of the hind-legs in running needing a firmer support. In Hyomoschus this is the case. The hind-limbs are provided with a cannon bone, while the metacarpals of the fore-feet are still free.
The number of dorso-lumbar vertebrae is less in the Artiodactyle than in the Perissodactyle Ungulates. Whereas the former have but nineteen, the latter have, as a rule, twenty-three such vertebrae.[[173]] The number of ribs varies from twelve (Camelus, Hydropotes) through thirteen (Cervus, Gazella) to fourteen in Dicotyles, Giraffa, etc.
The curious form of teeth known as "selenodont" is characteristic of the Artiodactyla, though only found well developed in the modern forms, and of those only in the Pecora. The more primitive forms had "bunodont" teeth with typically four tubercles (if we except the tritubercular and but little-known Pantolestes); and the intermediate "buno-selenodont" type characterises such groups as the Anthracotheriidae.
While the stomach of the Perissodactyles is always a simple sac, it is complicated, or shows signs of complication, in the Artiodactyles. That of the Hippopotamus is divided into two chambers; there are three in Tragulus, and four in the typical Ruminants such as Cervus, Ovis, etc.
Had we to deal only with the still living genera of Artiodactyles, it would be easy to sort them into two groups on the characters of the teeth; for the Pigs and Hippopotamus are provided with tubercular molars; they are bunodont. The Deer, Camels, Oxen, Giraffes, etc., have selenodont molars. Besides, the latter are "Ruminants," and have a more complicated stomach. The existing Chevrotains forbid a more trenchant division, since they are, as will be pointed out in due course, somewhat intermediate in structure; the feet are more Pig-like, and the stomach is not so typically Ruminant. In any case such a division is prevented by certain extinct families which are perhaps ancestral to both. They have teeth which are not quite bunodont and not quite selenodont. These teeth have been termed buno-selenodont or buno-lophodont.
The distribution of the living Artiodactyles presents us with some interesting facts. The vast preponderance of species occurs in the Old World—34 in America as against over 250 species
in Europe, Asia, and Africa. The Neotropical region has no Oxen, or Sheep, or Antelopes. The latter are confined to Africa, Asia, and certain parts of the Palaearctic region; they are vastly more prevalent in Africa, where they take the place of the totally absent Deer. The Pig tribe is almost entirely Oriental and Ethiopian in distribution, only one form, the European Wild Boar, ranging into the Palaearctic region; and the two species of Peccary are found in both North and South America. Broadly speaking, the Ethiopian region is the headquarters of the Artiodactyla. But the great island of Madagascar has but one form of Artiodactyle, a Pig of the genus Potamochoerus.[[174]]
Group I.—SUINA.
Fam. 1. Hippopotamidae.—The family Hippopotamidae contains of existing genera only Hippopotamus, for the Liberian dwarf Hippopotamus is not now regarded, as it was formerly, as the type of another genus, Choeropsis. The reasons for its former separation were the loss of the outer pair of incisors and the different proportions of various parts of the skull. This little Liberian animal has, however, been shown by Sir W. Flower[[175]] to possess the missing incisors occasionally; and as to the proportions of the skull, it is exceedingly common for small animals to vary from larger relatives in this way. Hence, considering the characteristic features of the Hippopotamus and the fewness of species, it seems unnecessary to divide it up further. We shall therefore only recognise one genus.