The existing members of this order can be readily grouped into the Hyracoidea, Proboscidea, Perissodactyla, and Artiodactyla, each of which divisions has quite the value of an order, and all of which are sharply marked off from each other. But as the discovery of so many fossil forms has to a great extent rendered these demarcations less sharp, it is better to regard all these groups as not more than sub-orders of a larger "Order" Ungulata. Even when this conclusion has been necessarily arrived at from a consideration of the more ancient groups of Ungulate animals, the definition of such an order remains a difficult matter for the systematist. For the earliest of these forms, more particularly the Ancylopoda, the Amblypoda, and the Condylarthra, whose peculiarities will be dealt with at length subsequently, are not by any means easily differentiated from the primitive Carnivorous mammals of that date, the Creodonta; these latter, moreover, fade into the Marsupials through the so-called Sparassodonta of Professor Ameghino. To confine ourselves to the Ungulates, we may perhaps define them as terrestrial animals with hoofs rather than claws or nails, and chiefly, if not entirely, vegetarian in habit. The teeth are bunodont or lophodont, the tendency to the production of the latter type being always marked. The walk, although plantigrade in the older types, becomes more and more digitigrade, except in such survivals from antiquity as _Hyrax_. There is, too, as we pass from the ancient types to the modern, a gradual perfection of the limbs as running {196} and not climbing or grasping organs; the number of toes becomes reduced, and culminates twice (in the horse and in the Litopterna) in one toe on each foot; at the same time the ulna becomes rudimentary and fuses with the radius, and the fibula in the hind-limb undergoes a like reduction. The clavicle is absent even in some of the oldest types; its presence in _Typotherium_[120] is highly remarkable. The tail too, an organ which is long in some of the early forms, gets short in their modern derivatives. [Illustration] FIG. 110.--An early Ungulate. _Phenacodus primaevus._ x 1/12. (After Osborn.) [Illustration] FIG. 111.--Series of metacarpals and metatarsals of Camelidae, to show secular and progressive increase in size. From left to right the species are _Protylopus petersoni_, _Poebrotherium labiatum_, _Gomphotherium sternbergi_, _Procamelus occidentalis_. F, Fore-foot; H, hind-foot; III, IV, third and fourth metapodials. (After Wortman.)