Other allied genera are _Merychyus_ and _Leptauchenia_. The former extends as far down as the Lower Pliocene, and is thus one of the newest forms of Oreodontidae. _Agriochoerus_[221] (Fig. 178) is placed in a separate sub-family from the types which have just been considered. It is Miocene in range. It differs from _Oreodon_ and its closer relatives by the fact that the orbit is open behind and not closed. The most remarkable fact about this creature is that the terminal phalanges of the digits (five in the fore- and four in the hind-feet) being pointed, seem to suggest their encasement with claws rather than hoofs. The pollex, though small, seems to have been opposable. As with other Oreodonts, the molars are selenodont. The premaxillae are toothless--at least in adults, for two teeth are present in the {332} young. There are several species. _Agriochoerus_, like _Oreodon_ and primitive Ungulates in general, had a long tail. The genus thus shows a mixture of ancient and specialised characters. [Illustration] FIG. 178.--Skeleton of _Agriochoerus latifrons_. x 1/8. (After Wortman.) The most ancient form of Oreodont is _Protoreodon_. This is Eocene, and became extinct during that period. It had a complete dentition, open orbit, and no lachrymal fossa. The fore-feet were five-toed, the hind four-toed. FAM. ANOPLOTHERIIDAE.--This family is entirely Eocene in point of time, and is unknown outside Europe. The dentition of the group is complete; the molars are seleno-bunodont, like those of the Anthracotheriidae. The bones of the carpus, tarsus, metacarpus and metatarsus are all free; the toes are four to two in number on each foot. The orbit is widely open behind. The tail is long, as in _Xiphodon_, etc. {333} These general characters only just serve to differentiate the family; but they illustrate its archaic character, in which it resembles the Xiphodontidae, and even more the Anthracotheriidae. A survey of some of the genera which have been assigned to the family will bring out other features in the organisation of these very ancient Artiodactyles. _Anoplotherium_ is so called on account of the fact that it is, like all ancient Artiodactyles, without horns or claws. Tusks it might have, but as a matter of fact has not. There are, as in Artiodactyles generally, nineteen dorso-lumbar vertebrae; the long tail has numerous chevrons. The shoulder blade has a well-marked acromion and a distinct coracoid process; it is wide proximally. The bones of the fore-arm and fore-leg are, as is usual in primitive Artiodactyles, separate.