i

3 - 3

; c

1 - 1

(or

0 - 0

) ; pm

3 - 3

; m

3 - 3

=

40

or

38.

3 - 3

1 - 1

0 - 0

3 - 3

3 - 3

The skin is covered with hair, and the neck is furnished with a mane.

As regards the dentition of the recent Equidae, there are sometimes praemolars, but the first praemolar usually disappears in adult life. The canines are of small size. The outer side of the molars (fig. 396) is deeply grooved, with two parallel sulci, to which internal ridges correspond, their length being very great, and the whole external surface being thickly coated with cement; while the enamel-ridges and folds of the crown are filled in with the same substance. The enamel covering the incisors is folded in at the crown, like the inverted finger of a glove, the tube thus formed being filled in with soft cement; and it is the wearing down of this with age which constitutes the "mark."

4 - 4

4 - 4

The family Equidae is divided by Dr Gray into two sections or genera : Equus, comprising the Horse; and Asinus comprising the Asses and Zebras. Many authorities, however, place all the existing forms under the single genus Equus.

The genus Equus is distinguished by the fact that the animal is not banded, and has no dorsal line; both the fore and hind legs have warts, and the tail is hairy throughout. The genus appears to contain no more than one well-marked species, if the Asses be excluded, and as far as living forms are concerned - namely, the Equus caballus. From this single species appear to have descended all the innumerable varieties of horses which are employed by man. The native country of the horse appears to have been Central Asia, but all the known wild individuals of the present day appear to be descendants of domestic breeds.

Fig. 396.   Grinding surfaces of the last praemolar and of the three true molars of the upper jaw of the Horse. (After Cuvier.)

Fig. 396. - Grinding-surfaces of the last praemolar and of the three true molars of the upper jaw of the Horse. (After Cuvier.)

The Ass (Asinus vulgaris) is characterised by the fact that there is always a distinct dorsal line, and the body is more or less banded; the fore-legs alone have warts, and the tail has a tuft of long hair at its extremity. The Ass is probably a native either of Northern Africa, or of South-western Asia, and it has been supposed to be the descendant either of the "Djig-getai" (Asinus hemionus), or the "Onager" (Asinus onager), both wild existing species; though a more probable stock for it is to be found in the Asinus taeniopus of Abyssinia. According to Lenormant, the Ass was domesticated in Egypt at the very earliest periods of its history, long before the introduction of the Horse; and it may therefore be the descendant of a wild African form. The striped and banded asses are known as Zebras and Quaggas, and are distributed over the greater part of Africa. Several genera (Anchitherium, Hipparion, Orohip-pus, Miohippus, Pliohippus, etc.) have been founded upon the remains of fossil Equidae. Many of these are of special interest, as showing an almost perfect series of gradations between a foot with three complete toes and a foot with only one complete digit. Some of them also exhibit other curious tran-sitional characters.

The most ancient type of the Equidae is the Eohippus of the Lower Eocene of North America, in which the fore-feet have four complete toes and a rudimentary pollex, while the hind-feet have three toes.

Orohippus is the next oldest known Equine genus, and comprises small mammals about as big as foxes, with the fore-feet four-toed (fig. 397, A), and the hind-feet three-toed. In the fore-foot, the pollex alone is wanting, but the middle toe is much the largest. The genus is from the Eocene of North America.

Section A Perissodactyla 484Section A Perissodactyla 485Section A Perissodactyla 486Fig. 397.   Skeleton of the foot in various forms belonging to the family of the Equidae: A, Foot of Orohippus, Eocene; B, Foot of Anchitherium, Upper Eocene and Lower Miocene ; C, Foot of Hipparion, Upper Miocene and Pliocene; D, Foot of Horse (Equus), Pliocene and Recent. The numerals indicate the numbers of the digits in the typical five fingered hand of Mammals. (After Marsh.)

Fig. 397. - Skeleton of the foot in various forms belonging to the family of the Equidae: A, Foot of Orohippus, Eocene; B, Foot of Anchitherium, Upper Eocene and Lower Miocene ; C, Foot of Hipparion, Upper Miocene and Pliocene; D, Foot of Horse (Equus), Pliocene and Recent. The numerals indicate the numbers of the digits in the typical five-fingered hand of Mammals. (After Marsh.)

In the Miocene Tertiary occur the genera Anchitherium, Miohippus, and Mesohippus, all of which have three toes to both feet. Mesohippus has an additional "splint-bone" (rudimentary metacarpal, or metatarsal) representing a fourth toe. Miohippus, about as big as a sheep, has the three toes sub-equal, and all touching the ground. Anchitherium (fig. 397, B) has the middle toe much the largest, though the lateral toes still reach the ground.

In the later Miocene and earlier Pliocene we find the genus Hipparion, in which the foot is still three-toed (fig. 397, C) ; but the middle toe is alone functionally useful, the two lateral toes, though appearing externally, not being long enough to touch the ground.

In the later Pliocene we meet with the genus Pliohippus, in which the foot is precisely that of Equus, with the lateral toes reduced to splint-bones (fig. 397, D), but there is an additional praemolar, and an "antorbital fossa" is present. Lastly, in the Post-pliocene appears the genus Equus itself.