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.
Of these, _Megalonyx_, _Mylodon_, _Hipparion_, _Platygonus_, _Anancus_, _Gomphotherium?_, _Stegomastodon_, and _Felis_ are known from the first interglacial stage. _Anancus_ includes mastodons with short, tuskless lower jaws and trefoiled molars. _Gomphotherium_, having long lower jaws with tusks, upper tusks with enamel band, and with trefoiled molars, may be represented by some of the early Pleistocene species. The same species of _Stegomastodon_ appears to be present in the Blanco as in the Pleistocene, _S. mirificus_. The edentate _Glyptotherium_ is not far removed from _Glyptodon_ of the early Pleistocene. The Blanco genera of horses are so close to _Equus_ that Cope regarded them as belonging to this genus.
The matter may be looked at from another point of view. If _Mylodon_, _Megalonyx_, and _Glyptotherium_ are referred to the Middle Pliocene, we shall probably have them recorded as living in Texas before they existed in South America. It is true that Santiago Roth (Neues Jahrb., Min. Beil., Bd., vol. XXVI, table opposite p. 144) states that _Glyptodon_ occurs in the Lower Pampas beds, and these he refers to the Upper Miocene; but the writer believes that Wilckens (Neues Jahrb, Min. Beil., Bd., vol. XXI, p. 193) is more nearly correct in placing them in the Pliocene. While the opinion may be correct that, when no obstacles intervene, the time required for mammals to spread over even a continent constitutes but a small part of a geological age, yet in making their way from South America, especially from Argentina, along the narrow bridge that appears to have been offered them, probably over mountain ranges, and across rivers and gorges, and in the face of the competing fauna advancing from the north, some of which were wolves and saber-tooth tigers, the slowly plodding and inoffensive edentates would have encountered too many hindrances to be able to make the journey in a short time.