B. Right Manus of Titanotherium. (After Marsh.)
C. Left Manus of Chalicotherium gigantium. (After Gervais.)
| 1. scaphoid. | 6. unciform. |
| 2. lunar. | 7. trapezium. |
| 3. cuneiform. | II, III, IV, V. second, third, |
| 4. trapezoid. | fourth and fifth digits. |
| 5. magnum. |
While the manus of the Artiodactyla is symmetrical about a line drawn between the third and fourth digits, that of the Perissodactyla is symmetrical about a line drawn through the middle of the third digit, which is larger than the others and has its ungual phalanx evenly rounded and symmetrical in itself. The most reduced manus in the whole of the mammalia is found in the Horse and its allies, in which the third digit, terminated by a very wide ungual phalanx, is the only one functional. Small splint bones representing the second and fourth metacarpals are attached to the upper part of the third metacarpal. In Hipparion[174] and other early horse-like animals the second and fourth digits, though very small and functionless, are complete and are terminated by small hoofs. In Rhinoceros the second and fourth digits are equally developed and nearly as large as the third, and reach the ground in walking, a vestige of the fifth is also present. In the Tapir (fig. 106, A) and Hyracotherium the fifth digit is fully developed but is scarcely functional. In Titanotherium (Brontops) (fig. 106, B) it is nearly as well developed as any of the others, and there is little or no difference between the relative development of the third and fourth digits.
The Chalicotheriidae[175], though distinctly Perissodactyles in various respects such as their cervical vertebrae and teeth, differ not only from all other Perissodactyles, but from almost all other Ungulates, in the very abnormal character of their manus. For while the carpus and metacarpus are like those of ordinary Perissodactyles, the phalanges resemble those of Edentates, each second phalanx having a strongly developed trochlea, and each distal one being curved, pointed and deeply cleft at its termination (fig. 106, C).
The Macraucheniidae, while agreeing with Perissodactyles in having only three digits, with the limb symmetrical about a line drawn through the middle of the third, have a carpus which approaches closely to the subungulate condition, the magnum articulating regularly with the lunar, and only to a slight extent with the scaphoid.
In the Subungulata the manus sometimes has five functional digits, and a considerable part of it rests on the ground in walking. The bones of the carpus retain their primitive relation to one another, the magnum articulating with the lunar, but not with the scaphoid. This character does not however hold in the Toxodontia, for in most of the animals belonging to this group the magnum does articulate with the scaphoid. The corner of the scaphoid just reaches the magnum also in Amblypoda.
As far as is known the Toxodontia generally have three, sometimes five digits to the manus, and the third is symmetrical in itself—a Perissodactyloid feature.
In Phenacodus (fig. 107, B) (Condylarthra) all five digits are well developed, the pollex being the smallest. The carpal bones retain their primitive arrangement, the magnum articulating with the lunar and not with the scaphoid. There is no separate centrale.
