MAMMALIA. 443 
Nothing is more striking than the characters by which it is distinguished 
from all others, namely, a single hoof with an apparently single toe to each 
foot; but on the skeleton, on each side of the metatarsus and metacarpus, 
there are spurs representing two lateral toes. Only:one genus of the pre- 
sent family is found in the actual fauna and with but comparatively few 
species, whilst in the tertiary deposits we find at least as many extinct 
species together with extinct genera. 
The genus Equus is characterized by six incisors in each jaw, the crowns 
of which, at an early age, are marked with a fossula, and six molars through- 
out, with a square crown, marked by lamine of enamel which dip into 
them, with four crescents, and in the upper ones, with a small disk on the 
inner edge. The male has also two small additional canines in the upper 
jaw, sometimes in both, which are almost always wanting in the female. 
Between these canines and the first molar is a free space corresponding to 
the angle of the lips where the bit is placed, and by which, as Cuvier 
remarks, man alone has been enabled to subdue and tame this powerful 
animal. 
The common horse, Equus caballus (pl. 110, fig. 4), is the most import- 
ant of all the animals that man has subdued. His associate in the chase, 
in war, and in the operations of agriculture, of arts and commerce, or raised 
for luxury, the horse has received by that perpetual contact a noble port 
and proud carriage. The horse, however, does not exist in a wild state at 
the present time, except in those places where horses were formerly domes- 
ticated and set at liberty, as in Tartary and America. They live in troops, 
each of which is conducted and defended by an old male. The young 
males are forcibly expelled as soon as they have reached the age of puberty, 
and follow the troop at a distance until they are joined by some of the 
younger mares. They propagate at four years; the period of gestation is 
eleven months. The age of horses is known by the incisors. The milk teeth 
begin to grow about fifteen days after the colt is foaled; at two years and 
a half the middle ones are replaced, at three and a half the two succeeding 
ones; at four and a half the outermost, or the corners. All these teeth, 
with an originally indented crown, gradually lose that mark by detrition. 
When seven or eight years old they are entirely effaced, and the horse is 
no longer marked. The life of the horse seldom extends beyond thirty 
years. This animal varies very much in size and color. The principal 
races exhibit sensible differences in the form of the head, in their propor- 
tions, and in their fitness for the various uses to which they are applied. 
1. The most beautiful and swift is the Arab (of which pl. 110, fig. 6, 
represents the mare with her colt, and fig. 7 the stallion), which has been 
instrumental in improving the Spanish race, and in connexion with the 
latter has contributed to form the English race; the Barbary, Persian, 
Circassian, and Turkish horses descend from the Arab. The Arab horse 
inhabits western Asia and northern Africa, where it is found of medium 
size. 
2. The northern horse, rather small than large, light, docile, swift, hard- 
working, enduring, and satisfied with little and common food. The Tartar, 
647 
