The glacial deposits of the Nebraskan stage are not as well known as one might wish. They appear to be in general overlain by the later drifts and are observed mostly where streams have cut through both the overlying drift and the Nebraskan. The old drift found in New Jersey is thin and of no great extent. Moreover, we can hardly expect to find fossil vertebrates in the drift itself. We must therefore depend on studies of supposed Nebraskan fossils found mostly outside of the glaciated area and make comparison of them with earlier and later faunas. If we shall discover collections of Nebraskan vertebrate animals, we may be sure that they will differ from those of the first interglacial, the Aftonian. We may be pretty certain that they will include autochthonous genera of the late Tertiary, which may be missing from the Aftonian, together with at least a few genera from South America and others from Asia.
Now, have any formations and included fossil vertebrates been found which may be fitted into the Nebraskan interval?
In this stage the writer places the beds which Cope designated the Idaho formation (Cope, Proc. Acad. Nat. Sci. Phil., 1883, p. 135). Since Cope’s time several new species have been added to his list from this formation. In 1917 (Bull. Dept. Geol. Univ. Calif., vol. X, p. 432), Dr. J. C. Merriam published a list of the fossils, except fishes, which had been secured up to that time. The list of species referred to the Idaho formation is as follows:
Equus idahoensis. E. excelsus? Protohippus? Rhinoceros, probably Aphelops (Teleoceras) fossiger. Mastodon mirificus. Cervus, possibly new. Smaller and more slender than C. canadensis. Procamelus, size of P. major. Tragocerus? horn-core of antelope. Ischyrosmilus n. sp. Morotherium leptonyx. Castor, possibly n. sp. Olor, size of O. paloregonus. Graculus idahoensis.
In this collection the presence of horses of the genus _Equus_, of _Cervus_, _Morotherium_, and _Castor_, is strongly suggestive of the Pleistocene. The type of _Mastodon mirificus_ was found in Pleistocene deposits of probably Aftonian age. Although rhinoceroses are supposed to have become extinct before the end of the Pliocene, this supposition may be an error. The list of Blanco vertebrates is a short one, and the absence of a genus from it is not decisive. One drawing of a seine in the sea-waters of Florida would furnish inadequate materials for conclusions about the fish fauna of that coast.