Fdm. 1. Hippo-potamidae. - This group contains only the single genus Hippopotamus, characterised by the massive heavy body, the short blunt muzzle, the large head, and the presence of teeth of three kinds in both jaws (fig. 400). The dental formula of the living Hippopotamus amphibius is:

i

2 - 2

; c

1 - 1

; pm

4 - 4

; m

3 - 3

=

40.

2 - 2

1 - 1

4 - 4

3 - 3

The incisors are nearly horizontal, those of the centre of the lower jaw being long and tusk-like. The canines are greatly developed, those of the upper jaw being comparatively short, while the lower canines are in the form of enormous tusks, with a chisel - shaped edge. The crowns of the praemolars and molars exhibit a characteristic double - trefoil pattern. The legs are very short, with massive feet, terminated by four hoofed toes each (fig. 401). The eyes and ears are small, and the skin is extremely thick, and is furnished with few hairs. The tail is very short.

Several extinct species of Hippopotamus are known; but there is only one familiar living form, the Hippopotamus amphibius or River-horse, and this is confined to the African continent. It is an enormously bulky and unwieldy animal, reaching a length of eleven or twelve feet. It is nocturnal in its habits, living upon grass, the foliage of trees, and herbs, and it swims and dives with great facility. It is found in tolerable abundance in the rivers of Abyssinia, and occurs plentifully in South Africa. A much smaller form (the so-called Hippopotamus or Chaeropsis Liberieusis) occurs on the west coast of Africa, but it is exceedingly rare, and comparatively little is known about it. It possesses, however, only two lower incisors instead of four.

Fig. 400.   Skull of Hippopotamus amphibius, side view. (After Giebel.)

Fig. 400. - Skull of Hippopotamus amphibius, side-view. (After Giebel.)

Fig. 401.   Left fore foot of Hippopotamus amphibius. (After Cuvier.)

Fig. 401. - Left fore-foot of Hippopotamus amphibius. (After Cuvier.)

Fam. 2. Suida. - The group of the Suida, comprising the Pigs, Hogs, and Peccaries, is very closely allied to the preceding ; but the feet (fig. 388) have only two functional toes, the other two toes being much shorter, and hardly touching the ground. All the three kinds of teeth are present, but they vary a good deal. The canines (fig. 402) always are very large, and trihedral in shape; and in the males they usually constitute formidable tusks projecting from the sides of the mouth. The incisors are variable, but the lower ones are always inclined forwards. The molars and premolars have broad crowns, with two transverse ridges (increased to three or more in the last molar), which are divided into rounded tubercles (fig. 398). The permanent dental formula of the Boar (Sus scrofa) is in the jaw up to the third year of life. If, therefore, the jaw of a Pig up to the third year of its age (fig. 403) be examined, there will appear to be four praemolars and three molars on each side, the first of these apparent praemolars being really the long - retained first deciduous molar. The stomach is mostly slightly divided, and is not nearly so complex as in the Ruminants. The snout is truncated and cylindrical, fitted for turning up the ground, and is capable of considerable movement. The skin is more or less abundantly covered with hair, and the tail is very short, or represented only by a tubercle.

Fig. 402.   Skull of the Wild Boar (Sus scrofa ferus). (After Gray.)

Fig. 402. - Skull of the Wild Boar (Sus scrofa ferus). (After Gray.)

i

3 - 3

; c

1 - 1

; pm

3 - 3

: m

3 - 3

=

40.

3 - 3

1 - 1

3 - 3

3 - 3

In the young animal there are four deciduous molars, but the first of these is not replaced by a praemolar, though it remains

Fig. 403.   Dentition of the Boar (Sus scrofa). The tooth marked pm1, though taking the place of a first praemolar, is really the first deciduous molar, which has not yet been shed.

Fig. 403. - Dentition of the Boar (Sus scrofa). The tooth marked pm1, though taking the place of a first praemolar, is really the first deciduous molar, which has not yet been shed.

Of the true Swine, the best known and most important is the Wild Boar (Sus scrofa), from which it is probable that most of our domestic varieties of swine have sprung. The Wild Boar formerly inhabited this country, and is still abundant in many of the forests of Europe. It is often hunted, and the size and sharpness of its canines render it a tolerably formidable adversary, as is also its congener, the Indian Hog (Sus Indicus). Another curious form, closely related to the Wild Boar, is the Babyroussa (Porcus Babirusa), which inhabits the islands of Celebes and Borneo in the Melanesian province. It is remarkable for the great size and backward curvature of the upper canines. The upper canines pierce the upper lip in the males, and their alveoli are directed upwards. The Bush-hogs (Potamochoerus) of Southern Africa and Madagascar are nearly allied to Sus, but possess sub-ocular excrescences of a cartilaginous nature.

The African Wart-hogs, forming the genus Phacochoerus, are distinguished by having a fleshy wart under each eye. They inhabit Abyssinia, the Guinea coast, and other parts of Africa. The American Peccaries (Dicotyles) represent the Swine of the Old World. They are singular for having only three toes on the hind-foot, the outer of the two supplemental digits being represented only by its metatarsal. The canines are not exserted, there are only four upper incisors, and there is no tail. They are exclusively confined to the American continent, extending from Paraguay as far north as Texas and Arkansas, and the commonest species is the Collared Peccary (Dicotyles torquatus). They are not at all unlike small pigs either in their appearance or in their habits, and they are gregarious, generally occurring in small flocks.

Fig. 404.   Anoplotherium commune. Eocene Tertiary, France. (After Cuvier.)

Fig. 404. - Anoplotherium commune. Eocene Tertiary, France. (After Cuvier.)

Fam. 3. Anoplotheridae. - This group comprises extinct Artio-dactyles which belong to the Eocene and Miocene periods, and form a kind of transition between the Swine and the Ruminants. In Anoplotherium itself (fig. 404) the body is slender, provided with a long tail, and having the feet terminated by two toes each, sometimes with small accessory hoofs in addition. The dentition is remarkable in the fact that no gap or diastema exists between the molars and the canines, the teeth thus forming an even and uninterrupted series. The dental formula is:

i

3 - 3

; c

1 - 1

; pm

4 - 4

; m

3 - 3

=

44.

3 - 3

1 - 1

4 - 4

3 - 3

Fam. 4. Oreodontidae. - This family comprises extinct Artio-dactyles from the Miocene and Pliocene Tertiary of North America, which stand in some respects midway between the Suida and the Ruminantia, and have been termed "Ruminating Hogs," though there is no evidence that they really ruminated. Oreodon is about as big as a sheep, the feet being four-toed, and the dental formula "complete." The canines are large, and triangular, and the molars are of the "seleno-dont" character, while there is the anomalous character that "larmiers" or "tear-pits" existed below the eyes.