1254 CLASS MAMMALIA. 



pre- and postaxial position in the Cetacea ; but in most other Mam- 

 mals they are crossed at their distal ends, so that their positions 

 become reversed, and in the " prone " or normal position of the limb 

 the radial, or preaxial border of the hand, becomes internal. In the 

 Primates, however, these bones admit of motion upon one another ; 

 and when the hand is " supine " (that is, with its palm directed for- 

 wards or upwards), the bones of the fore-arm occupy their original 

 primitive position. In the majority of those Mammals whose limbs 

 are adapted solely for walking, the ulna is more or less reduced, and 

 the radius, especially at its proximal end, is much enlarged, so that 

 it articulates with the whole of the anterior surface of the humerus ; 

 and thus comes in front of the ulna, instead of at its side. The 

 carpus essentially corresponds with that of the type represented in 

 fig. 829 (p. 907) ; the radiale, 1 intermedium, and ulnare being termed 

 the scaphoid, lunar, and cuneiform ; the trapezium, trapezoid, and 

 magnum representing the 1st, 2d, and 3d carpalia, and the unciform 

 the 4th and 5 th of that series. A centrale is present in the carpus 

 of embryos of pentedactylate forms ; but in the adult it usually 

 fuses with the scaphoid, although it remains distinct in most of 

 the Primates. In some groups others of these elements may 

 also coalesce, and one or more may be absent. The pisiform, or 

 presumed representative of the seventh digit, is generally well de- 

 veloped ; while in pen tadacty late types there may be an ossification 

 representing the prepollex. The metacarpals and digits may be five 

 in number (Proboscidea and Primates), or may be reduced to two, 

 or even to one functional member. Among the Ungulata, when 

 the metacarpus is reduced to a single functional element, as in the 

 Horse, such element is frequently termed the cannon-bone ; in ana- 

 tomy, however, this term is more usually restricted to the metapo- 

 dium of those Artiodactyla which consists of the coalesced third 

 and fourth metapodials. Except in the Cetacea there are never 

 more than three phalangeals to each digit, but by suppression or 

 anchylosis this number is occasionally reduced ; and the first digit 

 (pollex) has but two phalangeals. The pelvic girdle is also fully 

 developed in all Mammals except the Cetacea and Sirenia ; and in 

 the adult the three elements coalesce to form an innominate bone. 

 The pubis and ischium of the same side always unite to enclose the 

 obturator foramen ; and the two pubes meet in a ventral symphysis, 

 which is, however, not completely united in certain Insectivora. In 

 a large number of instances the ischia meet in a ventral symphysis, 



1 Since the preceding chapters were in type, Dr Baur has expressed his opinion 

 that the bone termed radiale in fig. 829 is really a second centrale ; and that the 

 radiale is represented by a minute bone generally known as the radial sesamoid. 

 The Mammalian scaphoid is accordingly also regarded as a second centrale. In 

 the same communication, Dr Baur expresses his disbelief in the existence of rem- 

 nants of a prepollex and of a seventh digit in Mammals and other Vertebrates. 



