shown by the presence of many fresh-water molluscan remains at certain horizons.
The fauna of the Barstow beds represents a stage in the evolution of Tertiary mammalian faunas previously not distinctly recognized in the Great Basin Province. It seems clearly later than the Middle Miocene life stage well known in the Mascall beds of Oregon and in the Virgin Valley beds of northern Nevada. The fauna is markedly older than the Rattlesnake Pliocene of Oregon and the Thousand Creek Pliocene of Nevada, representing the next described stages following the Middle Miocene in the Great Basin. The fauna of the Barstow beds has few if any species in common with that of the Ricardo formation, and is of a distinctly older type. Its nearest relationships are with the fauna of the Cedar Mountain region of southwestern Nevada, from which it possibly differs somewhat in stage.
The Second Fauna, the Ricardo Pliocene
The number of species represented in the Ricardo fauna is approximately equal to that found in the Barstow Miocene and the groups of animals represented are in general of the same type. Comparisons between these two faunas or life stages can therefore be made with some degree of satisfaction. Coupled with the fact that the Mohave and Ricardo faunas comprise an approximately equal representation of similar groups, it is a matter of interest to note the almost complete difference between the species represented in the two, and that with one or two possible exceptions the species of the Ricardo stage represent more specialized or more progressive stages of evolution than the corresponding types seen in the Barstow fauna.
As in the Mohave stage, we find the Ricardo collections consisting mainly of horses and camels, the horses furnishing the most important and most characteristic forms thus far known.
The Ricardo horses are of at least three types, of which the most common includes one or more species of the genus Hipparion. These are large, three-toed forms with the side-toes reduced and the grinding teeth large. They resemble to some extent one of the small species of the Barstow Miocene, but are much larger; the side-toes are more reduced; and the teeth were longer-crowned, heavier, and of more complicated structure. The Ricardo Hipparion differs from most of the species referred to this genus in America, and belongs to the true Hipparion type, which J. W. Gidley considers as characteristic of the Old World, in contrast to a New World form, Neohipparion. Many of the teeth of the Ricardo species are practically indistinguishable from these of Hipparion richthofeni, a species abundantly represented in the early Pliocene or late Miocene of China. It has generally been assumed that the Old World horses of the Hipparion type are descended from
