The Pleistocene is regarded as being equivalent to what is known as the Glacial period. It began with the deployment of the ice-sheets which, proceeding from their centers of accumulation in British America, laid down in the East the Jerseyan drift and in the West the Nebraskan. The more the Glacial period is studied the more one becomes impressed with the significance of its physical effects on the northern hemisphere and with its influence on the vertebrate life. Doubtless its effects on the world in general are only beginning to be comprehended. The writer knows of no other phenomena, geological or biological, which so well characterize the Pleistocene period as do those comprehended under the term Glacial. They constitute the key to the determination of the subdivisions of the epoch and of their succession and to the history of the vertebrates which during this time occupied the continent. II. THE BLANCO PLIOCENE. The Blanco is held to belong to the upper, or to the uppermost, Pliocene. It is at present assigned to the Middle Pliocene (Osborn, Bull. U. S. Geol. Surv. No. 361, p. 81; Matthew, ibid., p. 120). Until recently the oldest known Pleistocene vertebrates appeared to be represented by the collections which long ago were made at Fossil Lake, Oregon, and at Grayson (Hay Springs), Nebraska. These assemblages had formerly been referred to the Pliocene, and the belief that they belong there is not yet wholly without supporters. It seemed, therefore, proper to retire the Blanco somewhat. The discovery that the Fossil Lake and Grayson faunas were represented in the Aftonian deposits of Iowa, and belonged probably to the first interglacial stage, reveals the fact that the geological interval between the Blanco and the Aftonian is at least partly filled by the first glacial stage, the Nebraskan. Naturally, it is to be expected that the breach between the earlier and the later faunas will be occupied, in part at least, by the vertebrates of the Nebraskan. What these are is not yet well determined; but the writer believes that as the Blanco and its equivalent and closely related formations and faunas become better known, they will be attracted close to the Pleistocene. Aside from the facts just mentioned, the Blanco fauna seems to the writer to be more closely related to the Aftonian than has been supposed. The genera which occur in the Blanco are the following: Megalonyx. Mylodon. Glyptotherium. Hipparion. Pliohippus. Protohippus. Platygonus. Pliauchenia. Anancus. Gomphotherium. Stegomastodon. Felis. Amphicyon? Borophagus. Canimartes.