FIG. 121.--Bones of the manus A, of Tapir (_Tapirus indicus_). x 1/5. B, of Rhinoceros (_Rhinoceros sumatrensis_). x 1/5. C, of Horse (_Equus caballus_). x 1/8. _c_, Cuneiform; _l_, lunar; _m_, magnum; _p_, pisiform; _R_, radius; _s_, scaphoid; _td_, trapezoid; _tm_, trapezium; _u_, unciform; _U_, ulna; _II-V_, second to fifth digits; _V_ in B, and _II_ and _IV_ in C, represented by rudimentary metacarpals. (From Flower's _Osteology_.) These Ungulates derive their name, which is that given by the late Sir Richard Owen, from the fact that the middle digit of the hand and foot is pre-eminent. As will be seen from Fig. 121, the axis of {236} the limb passes through the third finger, which is larger than any of the others, and is symmetrical in itself. In this the present group contrasts with the Artiodactyla, where the axis is not "mesaxonic," but where there are two digits, on either side of the axis, which are symmetrical with each other. This arrangement of the limbs is highly characteristic, but appears to be not quite universal. In the Titanotheres, which form a group of the Perissodactyles, the fore-limbs are not quite accurately mesaxonic. Nor on the other hand can all Ungulates which show the Perissodactyle condition be safely included in the present group. The ancient Condylarthra and the Litopterna show precisely the same state of affairs. But other features in their organisation lead to their separation from the Perissodactyles, of which, however, the Condylarthra are probably ancestors. The Litopterna on the other hand, which possess even one-toed members like _Equus_, are believed to represent a case of parallelism in development. The number of functional toes varies from four to one. In the ankle joint the astragalus either does not, or does only to a comparatively slight extent, articulate with the cuboid as well as with the navicular bone. Moreover the fibula when present does not as a rule articulate with the calcaneum. In the opposed group of Artiodactyles the precise reverse of these conditions obtains. It is usually stated as part of the definition of this group that they do not possess horns of the type of those met with in the Cervicornia and Cavicornia. But the strong bony bosses on the skull of many Titanotheres, so curiously reminiscent of those of the not nearly related _Dinoceras_ and _Protoceras_, may well have supported horns of the Ox and Antelope pattern. [Illustration] FIG. 122.--Bones of the manus of Camel (_Camelus bactrianus_). x 1/8. _c_, Cuneiform; _l_, lunar; _m_, magnum; _R_, radius; _s_, scaphoid; _td_, trapezoid; _u_, unciform. (From Flower's _Osteology_.) The teeth of the Perissodactyles are lophodont, more rarely bunodont. The selenodont Artiodactyle form of molar is not met with. The dental formula, moreover, is at least near the {237} complete one, the more modern forms as usual being the more deficient in numbers of teeth.