Passing on to the Even-toed or Artiodactyle Ungulates, no representative of the Hippotamus seems yet to have existed, but there are several forms (Choeropotamus, Hyopotamus, &c.) more or less closely allied to the Pigs (Suida); and the singular group of the Anoplotheridoe may be regarded as forming a kind of transition between the Swine and the Ruminants.  The Anoplotheria (fig. 231) were slender in form, the largest not exceeding a donkey in size, with long tails, and having the feet terminated by two hoofed toes each, sometimes with a pair of small accessory hoofs as well.  The teeth exhibit the peculiarity that they are arranged in a continuous series, without any gap or interval between the molars and the canines; and the back teeth, like those of all the Ungulates, are adapted for grinding vegetable food, their crowns resembling in form those of the true Ruminants.  The genera Dichobune and Xiphodon, of the Middle and Upper Eocene, are closely related to Anoplotherium, but are more slender and deer-like in form.  No example of the great Ruminant group of the Ungulate Quadrupeds has as yet been detected in deposits of Eocene age.

[Illustration:  Fig. 231.—­Anoplotherium commune.  Eocene Tertiary, France. (After Cuvier.)]

Whilst true Ruminants appear to be unknown, the Eocene strata of North America have yielded to the researches of Professor Marsh examples of an extraordinary group (Dinocerata), which may be considered as in some respects intermediate between the Ungulates and the Proboscideans.  In Dinoceras itself (fig. 232) we have a large animal, equal in dimensions to the living Elephants, which it further resembles in the structure of the massive limbs, except that there are only four toes to each foot.  The upper jaw was devoid of front teeth, but there were two very large canine teeth, in the form of tusks directed perpendicularly downwards; and there was also a series of six small molars on each.  Each upper jaw-bone carried a bony projection, which was probably of the nature of a “horn-core,” and was originally sheathed in horn.  Two similar, but smaller, horn-cores are carried on the nasal bones; and two much larger projections, also probably of the nature of horn-cores, were carried upon the forehead.  We may thus infer that Dinoceras possessed three pairs of horns, all of which resembled the horns of the Sheep and Oxen in consisting of a central bony “core,” surrounded by a horny sheath.  The nose was not prolonged into a proboscis or “trunk,” as in the existing Elephants; and the tail was short and slender.  Many forms of the Dinocerata are known; but all these singular and gigantic quadrupeds appear to have been confined to the North American continent, and to be restricted to the Eocene period.

[Illustration:  Fig. 232.—­Skull of Dinoceras mirabilis, greatly reduced.  Eocene, North America. (After Marsh.)]