To the middle and lower Oligocene is referred the great White River formation of South Dakota, Nebraska, Wyoming, etc., which is divisible into three clearly marked substages. The White River contains the best-known fauna of all of the North American Tertiaries, for collecting in these beds has been carried on for more than sixty years, and a greater number of complete and nearly complete skeletons has been secured than from any of the other formations. It is plainly evident that a land-connection existed with the Old World, which was interrupted in the John Day, as is shown by the intermigration of characteristic forms; but some barrier, presumably climatic, prevented any complete interchange of mammals, and very many genera and even families remained confined to one continent or the other. The aspect of the White River fauna changes in accordance with the direction from which it is approached. If one comes to the study of it from the Eocene, it displays a very modern aspect, given by the almost complete disappearance of the archaic groups of mammals and by the great multiplication of genera and species belonging to the progressive orders. These genera, it is true, are all extinct, but many of them stood in an ancestral relationship to modern forms. On the other hand, if approached from the Miocene side, the White River mammals seem to be very ancient and primitive and very different from anything that now lives. We speak of horses and rhinoceroses, dogs and cats, in this fauna, but those terms can be employed only in a very wide and elastic sense to designate animals more or less distantly allied to those of the present day. [Illustration: FIG. 133.—1. _†Archæotherium._ 2. Ancestral camel (_†Poëbrotherium_). 3. _†Merycoidodon._ 4. _†Agriochœrus._ 5. Ancestral horse (_†Mesohippus_). 6. _†Hoplophoneus._ 7. _†Bothriodon._ 8. _†Hyænodon._ 9. †Cursorial rhinoceros (_†Hyracodon_). 10. _†Protoceras._ 11. Hornless rhinoceros (_†Cænopus_).]