It is necessary to determine, if possible, where the boundary line shall be drawn between the Pliocene and the Pleistocene. Room must be made for the first interglacial, the Nebraskan, and its fauna. How long this first glacial stage continued we do not know. Chamberlin and Salisbury have indicated (Geology, vol. III, p. 420) that in a rough way the dates from the present of the culmination of the various glacial stages, except the Nebraskan, taken in order backward, may be represented by the geometrical series 1, 2, 4, 8, 16. That is, if the Illinoian stage had its culmination 150,000 years ago, that of the Kansan occurred 300,000 years ago; if the Nebraskan should fall in the same series, it culminated 600,000 years ago; and it and the succeeding Aftonian interglacial held sway as long as all the rest of the Pleistocene put together. It would be rash to assert that this first glacial did last so long; but we see the possibilities. In a personal communication Professor Frank Leverett writes that he estimates that the Kansan culmination took place at not less than 400,000 years ago and the Nebraskan at 500,000. This, as the present writer estimates, would leave for the Nebraskan itself somewhere near 40,000 or 50,000 years. Some changes in the life of the Pleistocene must have been wrought during those years. 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: