2 6o THE POPULAR SCIENCE MONTHLY 



This difference extends in a considerable measure to groups of tlie 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 Eicardo 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 re- 

 semblance to that genus, but is not comparable to any Barstow species. 

 The heavy-jawed selurodons, which are the characteristic canids of the 

 Eicardo fauna, seem to be mainly, if not entirely, distinct, and are 

 generally more specialized than those from the Barstow beds. 



The fauna of the Eicardo 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 Eattlesnake Lower Pli- 

 ocene of Oregon, and is possibly somewhat older. 



The beds in which the Eicardo fauna occurs were evidently de- 

 posited 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 prob- 

 ably not greater than at present, and may have been somewhat less. 

 The Eicardo 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 Eicardo 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, Pliohip- 

 pus, 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 Eicardo 

 fauna which are necessarily considered as representatives of a forest 

 or mountain assemblage washed or carried out on the plains. 



The Eicardo fauna suggests climatic conditions permitting the de- 

 velopment 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 condi- 

 tions obtaining here in Eicardo time were probably more nearly like the 



