The most ancient known member of the family is the genus _†Eomoropus_, from the Bridger Eocene, which will be described by Professor Osborn in a paper soon to appear. _†Eomoropus_ was much nearer to the normal perissodactyls than were the genera from the Oligocene and Miocene above described. CHAPTER IX HISTORY OF THE ARTIODACTYLA The artiodactyls are and for a very long time have been a very much larger and more variegated group than the perissodactyls, and the Old World has been and still is their headquarters and area of special development, where they are represented in far greater number and variety than in the New; the perissodactyls, on the other hand, flourished especially in North America, as was shown in the preceding chapter. At the present time the artiodactyls are the dominant ungulate order, far outnumbering all the others combined, and include an assemblage of varied types, which, when superficially examined, appear to be an arbitrary and unnatural group. What could seem more unlike than a dainty little mouse-deer, no larger than a hare, a stag, a camel, a giraffe, a bison and a hippopotamus? Yet, in spite of this wonderful diversity of size, proportions, appearance and habits, there is a genuine unity of structure throughout the order, which makes their association in a single group altogether natural and proper, especially as these structural characters are not found united in any other group. It would be superfluous to enumerate all of the diagnostic characters which, on the one hand, unite all the living and extinct artiodactyls and, on the other, distinguish them from all other hoofed animals, and it will suffice to mention a few of the more significant of these features. As the name implies, the artiodactyls typically have an even number of toes in each foot, four or two; though this rule may be departed from and we find members of the order with five digits or three, just as the tapirs and nearly all the Eocene genera of perissodactyls had four toes in the manus. Much more important is the fact that the plane of symmetry, which in the perissodactyls bisects the third digit and is therefore said to be _mesaxonic_, passes between the third and fourth digit and is _paraxonic_. The third and fourth digits always form an equal and symmetrical pair and are the “irreducible minimum,” beyond which the number of toes cannot be diminished. A single-toed artiodactyl would seem to be an anatomical impossibility; at all events, such a monstrosity was never known. Hence the term “cloven” or “divided” hoof, which seems to take the solid hoof of the horse as the norm; but “cloven or divided,” while expressing the appearance of the foot with sufficient accuracy, is erroneous, if taken to mean the splitting of what was once continuous. [Illustration: FIG. 186.—Left fore-arm bones of the Domestic Pig (_Sus scrofa_). _R._, radius. _U._, ulna. _ol._, olecranon.]