In most mammals walking on four limbs, in which the hand is permanently prone, the ulna is much reduced in size, and the radius increased, especially at the upper end; so that the articular surface of the latter, instead of being confined to the external side of the trochlea of the humerus, extends all across its anterior surface, and the two bones, instead of being external and internal, are anterior and posterior. In many hoofed or Ungulate mammals, and in Bats, the ulna is reduced to little more than its upper articular extremity, and firmly ankylosed to the radius—stability of these parts being more essential than mobility. _Manus._—The terminal segment of the anterior limb is the hand or manus. Its skeleton consists of three divisions: (1) the “carpus,” a group of small, more or less rounded or angular bones with flattened surfaces applied to one another, and, though articulating by synovial joints, having scarcely any motion between them; (2) the “metacarpus,” a series of elongated bones placed side by side, with their proximal ends articulating by almost immovable joints with the carpus; (3) the “phalanges” or bones of the digits, usually three in number to each, articulating to one another by freely movable hinge-joints, the first being connected in like manner to the distal end of the metacarpal bone to which it corresponds. [Illustration: FIG. 17.—Dorsal surface of the right manus of a Water Tortoise (_Chelydra serpentina_). After Gegenbaur. U, Ulna; R, radius; _u_, ulnare; _i_, intermedium; _r_, radiale; _c_, centrale; 1-5, the five bones of the distal row of the carpus; _m¹_-_m⁵_, the five metacarpals.] _Carpus._—To understand thoroughly the arrangement of the bones of the carpus in mammals, it is necessary to study their condition in some of the lower vertebrates. Fig. 17 represents the manus in one of its fullest and at the same time most generalised forms, as seen in one of the Water Tortoises (_Chelydra serpentina_). The carpus consists of two principal rows of bones. The upper or proximal row contains three bones, to which Gegenbaur has applied the terms _radiale_ (_r_), _intermedium_ (_i_), and _ulnare_ (_u_), the first being on the radial or preaxial side of the limb.[11] The lower or distal row contains five bones, called _carpale_ 1, 2, 3, 4, and 5 respectively, commencing on the radial side. Between these two rows, in the middle of the carpus, is a single bone, the _centrale_ (_c_). In this very symmetrical carpus it will be observed that the _radiale_ supports on its distal side two bones, _carpale_ 1 and 2; the _intermedium_ is in a line with the _centrale_ and _carpale_ 3, which together form a median axis of the hand, while the _ulnare_ has also two bones articulating with its distal end, viz. _carpale_ 4 and 5. Each of the carpals of the distal row supports a metacarpal.