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University of California Publications in Geology [Vol. ll 



laid. The volcanic materials which they contain may at times have 

 accumulated rapidly, but seem in general to have been deposited so 

 slowly that the region was nearly continuously habitable. 



The Kicardo 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, Plio- 

 Mppus, 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 oreodonts might inhabit the plains or frequent 

 the border of the mountain area to the west. There are no elements 

 in the Bicardo fauna which are necessarily considered as representa- 

 tives of a forest or mountain assemblage washed or carried out to the 

 plains. 



The Bicardo fauna suggests climatic conditions permitting the 

 development of vegetation suitable for grazing animals. This indi- 

 cates a somewhat heavier growth of grass than is found in this 

 region at the present time. There is nothing in the constitution of 

 the fauna to suggest conditions radically different from those obtain- 

 ing in this region today, but the presumption is in favor of less 

 extreme aridity than is now known on the western border of the 

 Mohave Desert. The conditions prevailing in this region in Ricardo 

 time were probably not widely different from those now obtaining 

 in the southern portion of the Great Valley of California. 



Stage of Evolution and Relationships of the Ricardo Fauna 

 relation to tertiary faunas of the great basin province 



The fauna of the Ricardo beds is widely different from that of the 

 Middle Miocene Mascall and Virgin Valley, and is distinctly more 

 progressive or later than that of the Upper Miocene Barstow. It is 

 quite different from the Pliocene of Thousand Creek and is evidently 

 less advanced. It differs also so far as known from the Rattlesnake 

 Pliocene, and is presumably somewhat older. 



Comparison of the Ricardo and Barstow faunas as shown in the 

 following table shows almost complete specific separation of the two 

 life assemblages, and considerable difference in the genera, especially 

 in the Equidae, the best known group. 



