Several distinct lines of evidence go to prove that the junction of the Americas dates from the Miocene, possibly from the beginning of it. The absence of Atlantic species from the Pacific Miocene is an indication that the passage from ocean to ocean had been closed, and this is confirmed by the geology of the Central American and Isthmian region. In the middle Miocene of Oregon and Nebraska have been found remains, which are unfortunately too incomplete for altogether convincing identification, but which can be interpreted only as belonging to the extinct and most characteristically South American group of edentates, the †ground-sloths or †Gravigrada; if this reference is correct, the fact of the junction cannot be questioned. Continental deposits of Miocene date, chiefly accumulations made by rivers and the wind, cover vast areas of the western interior, though but rarely to any considerable depth. These have been divided into several stages and have received various names; the lower Miocene, known as the _Arikaree_, _Harrison_ or _Rosebud_, overlies the White River in South Dakota, western Nebraska and eastern Wyoming, with smaller areas in Montana and Colorado. In the deposits of this stage there are no mammals of indisputably Old World type, though a few which I consider to be such are a probable indication of renewed connection with Europe. The middle Miocene is found typically in central Montana, where it is called the _Deep River_ (or _Smith River_) stage, but occurs also in numerous small, scattered and widely separated areas in Oregon, Wyoming, Colorado and Texas, with local names in these different states. It is most likely that these middle Miocene formations are not strictly contemporaneous in the geological sense, but rather form a closely connected and successive series. The mammals of the Deep River stage leave no doubt that the way of migration from the Old World was again open. The _Loup Fork_, or upper Miocene, itself susceptible of further subdivision, is by far the most extensive of the Miocene formations and covers much of the Great Plains region, in separate areas, from South Dakota far into Mexico. Perhaps also referable to the upper Miocene is a small, but very interesting formation, the _Florissant_, which is in the South Park of Colorado; it was made by very fine volcanic material showered into a small and shallow lake. The finely laminated papery shales of the Florissant have preserved countless plants and insects and many fishes, and these throw very welcome light upon the vegetation and climatic conditions of the epoch and afford an interesting contrast to the fauna and flora of the Green River shales of the lower or middle Eocene. That the Florissant shales are Miocene, no one questions, but their isolated position and the fact that they have yielded no mammals make it somewhat doubtful whether they belong in the middle or later part of the epoch.