Ungulates as a distinct order, make their first appearance in North America in the lower Pliocene, where several species of Mastodon have been found. This genus occurs, also, in the upper Pliocene, and in the Post-Tertiary; although some of the remains attributed to the latter are undoubtedly older. The Pliocene species all have a band of enamel on the tusks, and some other peculiarities observed in the oldest mastodons of Europe, which are from essentially the same horizon. Two species of this genus have been found in South America, in connection with the remains of extinct llamas and horses. The genus Elephas is a later form, and has not yet been identified in this country below the upper Pliocene, where one gigantic species was abundant. In the Post-Pliocene, remains of this genus are numerous. The hairy mammoth of the Old World (Elephas primigenius) was once abundant in Alaska, and great numbers of its bones are now preserved in the frozen cliffs of that region. This species does not appear to have extended east of the Rocky Mountains, or south of the Columbia River, but was replaced there by the American elephant, which preferred a milder climate. Remains of the latter have been met with in Canada, throughout the United States, and in Mexico. The last of the American mastodons and elephants became extinct in the Post-Tertiary.
The order Toxodontia includes two very peculiar genera, Toxodon and Nesodon, which have been found in the Post-Tertiary deposits of South America. These animals were of huge size, and possessed such mixed characters that their affinities are a matter of considerable doubt. They are thought to be related to the Ungulates, Rodents, and Edentates; but, as the feet are unknown, this cannot at present be decided.
Macrauchenia and Homalodontotherium are two other peculiar genera from South America, now extinct, the exact affinities of which are uncertain. Anoplotherium and Palæotherium, so abundant in Europe, have not been found in our North American Tertiary deposits, although reported from South America.
Perhaps the most remarkable mammals yet found in America are the Tillodontia, which are comparatively abundant in the lower and middle Eocene. These animals seem to combine the characters of several different groups, viz., the Carnivores, Ungulates, and Rodents. In the genus Tillotherium, the type of the order, and of the family Tillotheridæ, the skull resembles that of the bears; the molar teeth are of the ungulate type, while the large incisors are very similar to those of Rodents. The skeleton resembles that of the Carnivores, but the scaphoid and lunar bones are distinct, and there is a third trochanter on the femur. The feet are plantigrade, and each had five digits, all with long, pointed claws. In the allied genus Stylinodon, which belongs to a distinct family, the Stylinodontidæ, all the teeth were rootless. Some of these animals were as large as a tapir. The genus
