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University of California Publications in Geology [Vol. ll 



In nearly all cases in which it has been possible to make a satis- 

 factory comparison of forms representing similar groups in the two 

 faunas, the Ricardo types are seen to be more specialized. In the 

 Carnivora the common Tephrocyon of the Barstow seems to have 

 disappeared. Specimens showing some resemblance to Tephrocyon 

 are not clearly comparable to any Barstow species. The aelurodons, 

 which are the characteristic canids of the Ricardo fauna, seem to be 

 mainly, if not entirely distinct, and are generally more specialized. 



In the Equidae the abundant Merychippus fauna accompanied by 

 rare Hypohippus, Parahippus, and possibly Protohippus or Plio- 

 hippus of the Barstow is replaced in the Ricardo by a group consisting 

 of specialized Hipparion and Pliohippus species which may, in part, 

 be derived from the earlier Barstow types. 



The only oreodont of the Ricardo is a Merycochoerus-like form 

 specifically distinct from that of the Barstow and more advanced. 

 The Ricardo camels include larger forms than those of the Barstow. 

 The known difference between the Merycodus forms is small, as only 

 fragmentary comparative material is available, but so far as deter- 

 minable the Ricardo species seems more specialized. 



The general correspondence in groups but difference in species 

 between the Ricardo and Barstow leads one to consider as probable 

 the derivation of a considerable part of the Ricardo fauna from stocks 

 near those represented in the Barstow. If this is true the difference 

 in stage of evolution indicates that a considerable time must have 

 elapsed between the deposition of the beds in which these faunas occur. 

 The amount of difference shown in comparison of these faunas does 

 not represent less than one-third of the change ordinarily taking 

 place within the limits of a geological period. If the Barstow is recog- 

 nized as Upper Miocene, which seems unavoidable, it is difficult to 

 consider placing the Ricardo lower than the base of the Pliocene, or 

 at the lowest, in the extreme upper limit of the Miocene. 



In comparison with the Cedar Mountain or Esmeralda fauna of 

 western Nevada the Ricardo shows much the same relationship as to 

 the Barstow. A few very fragmentary horse teeth from the Cedar 

 Mountain region suggest the presence of a type more advanced than 

 those of the Barstow and nearer the Ricardo forms, but the evidence 

 is as yet unsatisfactory. It is possible that in addition to the typical 

 Cedar Mountain fauna a later faunal stage is present in the Cedar 

 Mountain region. 



