The radius and the ulna, which together constitute the fore-arm, are both present in a large number of mammals, but the ulna tends to vanish in the purely walking and digitigrade Ungulates, being present, however, in the more ancient forms of these Ungulates. In Man and in many other mammals the radius can be moved from its normal position and crossed over the ulna; this movement of pronation has been permanently fixed in the Elephant, where the bones are crossed but cannot be altered in position by the contractions of any muscles. Other types agree with the Elephant in this fixation of the two bones. [Illustration] FIG. 31.--Bones of fore-arm and manus of Mole (_Talpa europaea_). x 2. _c_, Cuneiform; _ce_, centrale; _l_, lunar; _m_, magnum; _p_, pisiform; _R_, radius; _rs_, radial sesamoid (falciform); _s_, scaphoid; _td_, trapezoid; _tm_, trapezium; _U_, ulna; _u_, unciform; _I-V_, the digits. (From Flower's _Osteology_.) The bones of the wrist show great variation among mammals. The greatest number present are to be seen in such a type as the Mole. Here we have a proximal row, consisting of the scaphoid, lunar, cuneiform, and pisiform, which are arranged in their proper order, beginning with that on the radial side of the limb, that side which bears the first digit. A second row articulates proximally with these bonelets and distally with the metacarpals; the bones composing it are, mentioning them in the same order, trapezium, trapezoid, centrale, magnum, unciform. The centrale does not, however, really belong to the distal carpal row, and is as a rule situated in the middle of the carpus away from articulation with the metacarpals. It is a bone which is not commonly present in the mammalian hand, but is present in various lower forms, such as the Beaver and Hyrax. It also occurs in such high types as the majority of Monkeys; it is to be found in the Human foetal carpus. Many extinct forms possessed a separate centrale. Its importance in the formation of the interlocking condition of the Ungulate foot is referred to later, {41} on p. 196. The only mammal which appears to have the proper five bones in the distal row of the carpus corresponding to the five metacarpals is _Hyperoodon_, where this state of affairs at least occasionally occurs. The final bone of that series, the unciform, seems to represent two bones fused. Very often the carpus is reduced by the fusion of certain of the carpal bones; thus among the Carnivora it is usual for the scaphoid and the lunar to be fused. It is interestingly significant that these bones retain their distinctness in the ancestral Creodonts. In many Ungulates the trapezium vanishes. The reduction of the toes in fact implies a reduction of the separate elements of the carpus.