Dryptodon has been found only in the Coryphodon beds of New Mexico, while Tillotherium and Stylinodon occur in the middle Eocene of Wyoming. Anchippodus probably belongs to this group, which may perhaps include some other forms that have been named from fragmentary specimens.
The Rodents are an ancient type, and their remains are not unfrequently disinterred in the strata of our lowest fresh-water Eocene. The earliest known forms are, apparently, all related to the squirrels; and the most common genus is Sciuravus, which continued throughout the Eocene. A nearly allied form, which may prove to be the same, is Paramys, the species of which are larger than those of the older type. In the Dinoceras beds, the genus Colonomys is found, and the specimens preserved point to the Muridæ as the nearest living allies. A peculiar genus, Apatemys, which also occurs in the middle Eocene, has gliriform incisors; but the molars resemble those of Insectivores. All the Eocene Rodents known are of small size, the largest being about as large as a rabbit.
In the middle and upper Miocene lake-basins of the West, Rodents abound, but all are of moderate size. The hares first appear in the Oreodon beds, and continue in considerable numbers through the rest of the Tertiary and Post-Tertiary, to the present day. In these beds, the most common forms belong to the Leporidæ, and mainly to the genus Palæolagus. The Squirrel family is represented by Ischyromys, the Muridæ by the genus Eumys, and the beavers by Palæocastor. In the upper Miocene of Oregon, most of the same genera are found; and with them some peculiar forms, very unlike anything now living. One of these is the genus Allomys, possibly related to the flying-squirrels, but having molar teeth somewhat like those of the Ungulates. In the Pliocene, east and west of the Rocky Mountains, Rodents continue abundant; but most of them belong to existing genera. Among these are Castor, Hystrix, Cynomys, Geomys, Lepus, and Hesperomys. In the Post-Tertiary, the gigantic beaver, Castoroides, was abundant throughout most of North America. Hydrochœrus has been found in South Carolina. In the caves of the island of Anguilla, in the West Indies, remains of large extinct Rodents, belonging to the Chinchillidæ, have been discovered.
The early Tertiary Rodents known from South America are the genera Megamys, Theridromys, and a large species referred to Arvicola. In Brazil, the Pliocene Rodents found are referred to the existing genera, Cavia, Kerodon, Lagostomus, Ctenomys, Hesperomys, Oxymycterus, Arvicola, and Lepus. A new genus, Cardiodus, described from this horizon, is a true rodent; but the peculiar Typotherium, which has been referred to this order by some authorities, has perhaps other affinities. In the Post-Tertiary, the Rodents were very abundant in South America, as they are at present. The species are, in most instances, distinct from those now living, but the genera are