This difference extends in a considerable measure to groups of the rank of genera; as in the case of the horses, in which Hipparion replaces Merychippus. As has been noted above, in nearly all cases in which it has been possible to make a satisfactory comparison of animals in similar groups, the Ricardo types are seen to be more specialized or more progressive. In the Carnivora the common Tephrocyon of the Mohave seems to have disappeared. A single specimen shows some resemblance to that genus, but is not comparable to any Barstow species. The heavy-jawed ælurodons, which are the characteristic canids of the Ricardo fauna, seem to be mainly, if not entirely, distinct, and are generally more specialized than those from the Barstow beds.
The fauna of the Ricardo beds is widely different from that of the Middle Miocene west of the Wasatch, and is distinctly more advanced in the stage of progress or evolution. It is quite different from the Lower Pliocene of Thousand Creek of Northern Nevada, and seems less advanced. It differs so far as known from the Rattlesnake Lower Pliocene of Oregon, and is possibly somewhat older.
The beds in which the Ricardo fauna occurs were evidently deposited on plains lying at the eastern base of a Pliocene Sierra range rising to a height of several thousand feet above the level of the Great Basin region. The elevation of the Mohave area as a whole was probably not greater than at present, and may have been somewhat less. The Ricardo deposits are probably in part land-laid and in part water-laid. The volcanic material which they contain may at times have accumulated rapidly, but seems in general to have been deposited so slowly that the region was nearly continuously habitable.
The Ricardo fauna consists largely of forms that would naturally prefer to inhabit plains areas, or might thrive in partly open, level regions at least as well as in other environment. Hipparion, Pliohippus, the camels, and Merycodus would find this a favorable habitat. The carnivores associated with them would not necessarily find the surroundings unfavorable, provided sufficient cover were available. The mastodons and oreodons might inhabit the plains or frequent the border of the mountain area to the west. There are no elements in the Ricardo fauna which are necessarily considered as representatives of a forest or mountain assemblage washed or carried out on the plains.
The Ricardo fauna suggests climatic conditions permitting the development of vegetation suitable for grazing animals. This indicates a somewhat heavier growth of grass than is found in the Mohave at the present time. There is nothing in the constitution of the fauna to suggest conditions radically different from those obtaining in this region to-day, but the presumption is in favor of less extreme aridity than is now known on the western border of the desert. The conditions obtaining here in Ricardo time were probably more nearly like the
