The two genera above described, representatives of two distinct phyla within the family, held over, as it were, from the upper Miocene without essential change. The phylum of the hypsodont and slender _†Merychyus_ went back, with only minor modifications, into the upper substage of the lower Miocene, but cannot as yet be traced to an Oligocene ancestry; it is therefore still impossible to say just where and when it branched off from the main stem of the family. Future discoveries in the Oligocene will no doubt clear up this problem. The real terminal and most highly specialized member of the _†Merycochœrus_ phylum and the most extraordinary member of the entire family was confined to the upper Miocene. The extreme peculiarity of this genus (_†Pronomotherium_) was displayed only in the head, which was an exaggeration of the _†Merycochœrus_ type, the face being excessively shortened and the nasals so reduced as to show that the proboscis was much better developed than in the parent genus. The shortening of the face and the great vertical height of the skull and lower jaw gave a decided likeness to the skull of a great ape, though the proboscis would mask any such resemblance in the living head. _†Merycochœrus_ itself went back to the upper division of the lower Miocene, but in the lower division it was replaced by an ancestral genus, _†Promerycochœrus_, which had an elongate face and jaws and no proboscis; but in other characteristic features, such as the extreme thickness and roughness of the zygomatic arches, it was like its descendant. _†Promerycochœrus_ contained the largest known species of †oreodonts, some of them equalling a Wild Boar in stature, and its remains are found so abundantly in the middle and lower Miocene and upper Oligocene, that there must have been great herds of these animals over the plains. Probably it was itself derived from some of the larger species of _†Eporeodon_ of the upper White River beds, but there is a gap in the history, due to the fact that the lower part of the John Day is almost barren of fossils and the connecting link has not been recovered. [Illustration: FIG. 198.—Head of _†Pronomotherium laticeps_, upper Miocene. Restored from a skull in the Carnegie Museum, Pittsburgh.] It is an interesting and significant fact that ancestral and derivative genera may continue to live side by side in the same region. _†Promerycochœrus_, it is believed, gave rise to _†Merycochœrus_, but survived with it into the middle Miocene. _†Merycochœrus_, in its turn, produced _†Pronomotherium_, and, so far from being replaced by the latter, actually outlived it and persisted into the lower Pliocene. [Illustration: FIG. 199.—_†Promerycochœrus carrikeri_, lower Miocene. Restored from a skeleton in the Carnegie Museum, Pittsburgh.]