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NATURAL HISTORY. 



theridse, or Palaeotheres (iraXai&s, old ; drtpiov, beast) ; and (2) the Macraucheniadse (nanp6s, long ; a.\>x^v, 

 neck). In all the animals belonging to the group, the number of dorso-lumbar vertebrae is not fewer 

 than twenty-two, the third or middle digit of each foot is symmetrical, the femur or thigh-bone has 

 a third trochanter, or knob of bone on the outer side, and the two facets on the front of the asti'agalus 

 or ankle-bone are very unequal. When the head is provided with horns, they are skin-deep only, with- 

 out a core of bone, and they are always placed in the middle line of the skull, as in the Rhinoceros. 



In the Perissodactyla the number of toes is reduced to a minimum. Supposing, for example, we 

 compare the foot of a Horse with one of our own hands, we shall see that those parts which correspond 

 with the thumb and little finger are altogether absent, while that which corresponds with the middle 

 finger is largely developed, and with its hoof, the equivalent of our nail, constitutes the whole foot. 

 The small splint bones, however, resting behind the principal bone of the foot represent those portions 

 (metacarpals) of the second and fourth digits which extend from the wrist to the fingers properly so 

 called, and are to be viewed as traces of a foot composed of three toes in an ancestral form of the Horse, 

 which we shall discuss presently. In the Tapir, the hind foot is composed of three well-developed toes, 

 corresponding to the three middle toes in man, and in the Rhinoceros both feet are provided with three 

 toes formed of the same three digits. In the extinct Palteotherium also, the foot is constituted very 

 much as in the Rhinoceros. 



FAMILY I. EQUIDJE, THE HORSE-TKIBE. 



The Equidse, or Horse-tribe, comprise several living and many extinct species. Three living 

 members are restricted, in a state of nature, to Asia and Africa, and are divided into the true Horses, 

 which have horny patches or callosities on the inner sides of both pairs of limbs above the wrist in 

 the fore, and on the inner side of the metatarsus on the hind limbs and the Asses, which possess such 

 callosities only on the fore-limb. With the latter are classed the Zebras and the Quaggas. All the 



