112 THE MAMMALS. 



termed the acetabulum. The leg has two parallel bones 

 articulating with the lower end of the thigh-bone or 

 femur; of which the larger or tibia, occupying the inner 

 side of the limb, corresponds to the radius of the fore- 

 arm, while the smaller outer bone or fibula, represents the 

 ulna. The ankle, or tarsus, corresponds to the carpus in 

 the fore-limb, and likewise consists of two transverse rows 

 of small bones. Two bones of the uppermost row, viz. the 

 calcaneum or heel-bone, and the astragalus or ankle-bone, 

 are specially modified. In the foot proper the bones cor- 

 respond with those of the hand; those representing the 

 metacarpals being, however, termed metatarsals. 



"Most of the mammals have five fingers and toes, or 

 digits, on each foot or hand, but in some cases there is a 

 tendency to the reduction of the number of digits; cattle 

 and deer having only two, corresponding to the third and 

 fourth of the t}^pical series of five ; while in the horse only 

 a single digit remains, which in the fore-limb corresponds 

 to the middle or third finger of the human hand, and in 

 the hind-limb to the middle toe. 



"Almost all Mammals) when adult have both jaws 

 provided with a series of teeth varying greatly in num- 

 ber and structure in the different groups. These teeth 

 are almost invariably fixed in separate sockets; and while 

 the front teeth have but a single root or fang, the side or 

 cheek-teeth very generally have two or more such roots, 

 each of which occupies a separate division of the socket. 

 In all cases the teeth are fixed in their sockets merely by 

 the aid of soft tissues connected with the gum and are 

 never welded to the jaws by a deposit of bone. Very gen- 

 erally there is a sharply-marked line of division, termed 

 the neck, between the root, or portion of the tooth im- 

 planted in the jaw, and the crown or exposed portion. 



"In most of those Mammals in which the teeth of dif- 

 ferent parts of the jaw differ in structure from one an- 

 other, there are two distinct sets of teeth developed dur- 

 ing life. The first of the two includes the milk or baby 

 teeth, which are generally shed at a comparatively early 

 age. They are of small size and few in number, and are 

 finally succeeded by the larger and more numerous perm- 

 anent set, which remain during the rest of life, unless 

 previously worn out. 



