. The Cambridge natural history. Zoology. !36 FOOT OF UNGULATES the liml i passes through the thuxl 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 aiTangement 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 mesax- onic. Nor on

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. The Cambridge natural history. Zoology. !36 FOOT OF UNGULATES the liml_i passes through the thuxl 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 aiTangement 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 mesax- onic. Nor on the other hand can all Ungu- lates 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 theix 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 repre- sent a case of parallelism in development. The number of functional toes varies from four to one. In the ankle joint the astra- galus either does not, or does only to a comparatively slight extent, articulate with the cuboid as well as with the navicular 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 obtains. It is usually stated 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 remin- iscent of those of the not nearly related Dinoccras and Proto- ceras, may well have supported horns of the Ox and Antelope pattern. The teeth of the Perissodactyles are lophodont, more rarely liunodont. The selenodont Artiodactyle form of molar is not met with. The dental formula, moreover, is at leas