312 VERTEBRATE ANIMALS. 



cylindrical bones, articulatiiig proxiraally with the carpus, and. 

 distally with the phalanges of the fingers. The most remarkable 

 modification of this normal state of things occurs in the Euniinants 

 and in the Horse. In the Ruminants (fig. 222, A and B), in which 

 the foot is cleft, and consists of two perfect toes only, there are two 

 metacarpal bones in the embryo ; but these are ancliylosed together 

 in the adult, and form a single mass which is known as the " canon - 

 bone " (fig. 222, c). In the Horse (fig. 223), in which the foot con- 

 sists of no more than a single digit, there is only a single meta- 

 carpal bone, on each side of which are two little bony spines — 

 the so-called " splint-bones " — which are attached superiorly to the 

 carpus, and are to be regarded as rudimentary metacarpals. 



The normal number of digits is five, but they vary from one to 

 five. The middle finger is the longest, and most persistent of the 

 digits of the fore-limb ; and in the Horse it is the only one which 

 is left (fig. 22.3). The thumb is very frequently absent. In the 

 Kuminants there are only two fingers which are functionally useful, 

 these carrying the hoofs. Normally, each digit is composed of three 

 short bones or phalanges, the innermost digit (the thumb) having 

 two. The extremity of the digit is usually protected by a nail, 

 claw, or hoof. 



While the fore-limbs are never wanting, the hind-limbs are 

 absent in the so-called "Mutilated Mammals" (namely, the Ceta- 

 ceans and .Sirenians). The bones of the hind -limb correspond 

 with those of the fore-limb ; and the foot (pes) corresjDonds with 

 the hand {manus), consisting normally of five digits, but subject 

 to the same abortion of parts as occurs in the case of the anterior 

 limb. 



Teeth are present in the great majority of Mammals ; but they 

 are only pi'esent in the embryo of the whalebone Whales, and are 

 entirely absent in the genera Echidna, Munis, and Myrmecophaga. 

 In the Duck- mole (Ornithoi-hi/nchvs) the so-called teeth are horny, 

 and the same was the case in the extinct Rhytina amongst the 

 Sirenia. In all other Mammals the teeth have their ordinary 

 structure of dentine, enamel, and crusta petrosa, these elements 

 being variously disposed in difTerent cases, and the enamel being oc- 

 casionally wanting. In no Mammals are the teeth ever anchylosed 

 with the jaw ; and in all, the teeth are implanted into distinct 

 sockets or alveoli, which, liowever, are very imperfect in some of 

 the Cetacea. 



Many Mammals have only a single set of teeth throughout life, 

 and these are termed by Owen " mono])hyodont." In most cases, 

 however, the first set of teeth — called the " milk " or " deciduous " 

 teeth — is replaced in the course of growth by a second set of 



