The most ancient and primitive representative of the true rhinoceroses so far discovered occurs in the lowest division of the White River beds and is of particular interest as throwing light upon the origin of the family. The genus (_†Trigonias_) differed from that (_†Cænopus_) which was so abundant in the middle White River substage in several highly significant particulars, though on a merely casual inspection one might easily be misled into thinking that the two animals were nearly identical, for _†Trigonias_ was an undoubted rhinoceros. Such an identification, however, would be a great mistake, for the differences, though not striking, are very important. In the upper jaw the first or anterior incisor had already assumed the characteristic trenchant, chisel-like shape, but two other incisors were present also, thus bringing the number up to the original three, common to all early perissodactyls; even more interesting is the presence of a small upper canine. The lower jaw likewise had three incisors on each side, the first and third small, the second enlarged and tusk-like, but the canine had already been suppressed, and thus the dental formula was: _i_ 3/3, _c_ 1/0, _p_ 4/4, _m_ 3/3, × 2 = 42, or 14 more than the formula of the existing African species. The premolars were smaller and less complex than the molars. [Illustration: FIG. 176.—Skull of _†Trigonias osborni_, lower White River. (After Hatcher.)] [Illustration: FIG. 177.—Anterior end of right upper jaw of _†Trigonias osborni_ (after Lucas). _c._, canine. _i 3_, external incisor. _i 2_, middle incisor. _i 1_, first incisor.] From this ancient genus may readily be inferred the steps by which the peculiar characters of the anterior teeth in the true rhinoceroses were attained. The first stage was undoubtedly an animal in which, as in all other Eocene perissodactyls, there were three well-developed incisors on each side of both jaws, 12 in all, and moderately prominent canine tusks; all these teeth were erect. The second stage was the enlargement of the first upper and second lower incisors, the latter becoming less erect and beginning to assume the recumbent position; at the same time the other incisors and the canines were reduced in size and were so little used that they lost their functional importance. The third stage, in which the first and second lower incisors were horizontal and pointed directly forward, and the first upper and second lower teeth were still further enlarged, the non-functional teeth reduced in size and the lower canine suppressed, was realized in the genus _†Trigonias_. There were thus but two hypothetical stages between this lower White Region genus and the tapir-like forms of the middle Eocene, so far, at least, as the anterior teeth are concerned. [Illustration: FIG. 178.—Anterior end of left upper jaw of _†Cænopus_, _A_, adult; _B_, immature animal (after Osborn). _I 1_, first incisor; _I 2_, second incisor; _C_, canine.]