North American stock. No types from which Hipparion might presumably be immediately derived by evolution are known in the Old World formations of the period just anterior to that in which Hipparion first appears, whereas in North America stages of evolution leading toward Hipparion are found in formations representing the period preceding the birth of this genus. So far as the writer's observations have been carried, an evolutionary sequence leading to the genus, Hipparion is nowhere more clearly suggested than in the relation of the Hipparion of Ricardo to the Hipparion-like Merychippus of the Barstow Miocene. It seems not improbable that the Old World Hipparion is derived from a West-American form near the Barstow Merychippus.
Living in the same region with the Hipparion in Ricardo time were at least two other types of horses of an advanced stage referred to the genus Pliohippus. The animals of these species were nearly as large as the smaller forms of the modern domestic horse. Their teeth were long crowned and well adapted to grazing as in existing forms, but their feet still bore small side-toes somewhat as in Merychippus of the Barstow. The pattern of their teeth is quite unlike that of the Hipparion and considerable differences separate them in skeletal structure. They presumably occupied a different niche in the organization of the fauna, but what it was is not entirely clear.
In the Ricardo fauna, as at Barstow, we find a rare oreodon, the last representative of this important family known west of the Wasatch. The Ricardo type follows the rule in being more specialized than that in the Barstow Upper Miocene. Little deer-antelope much like those of Barstow are also known by the last representatives in the Great Basin. Rodents are rare. The mastodon group is still represented by animals with four tusks, a pair being present in the lower jaw as well as one in the upper jaw.
Of the camels there are several species known from Ricardo. They represent genera similar to those in the Barstow Miocene, but are generally of larger type, and are presumably in a large part specifically different. Carnivores are relatively abundant. Large heavy-headed ælurodons like those of Barstow are present, but possibly all belong to new species. With these are other forms of the same group, but larger and stronger. There is a marten of a new species. Of the cats, one is a saber-tooth of a rare type somewhat similar to a species known in India. One specimen, belonging to a gigantic animal of the Felis or true cat type, was at least as large as a male African lion of the present day. Another specimen is from a smaller cat possibly like a puma.
Large tortoises are known in the Ricardo, as at Barstow. At least one form seems to differ in its character from the Barstow species.
In the table on page 262 a comparison of the Ricardo and Barstow faunas would show almost complete specific separation of the life stages.
