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University of California Publications in Geology [Vol. ll 



It is probable, bowever, that as these characters disappear others not 

 previously observed will present themselves. 



In many respects Hipparion mohavense callodonte of the Ricardo 

 Hipparion group resembles a type from the island of Samos, which 

 has been referred to H. gracile. In this Samos form the protocone 

 is much more strongly compressed than in typical H. gracile, and 

 exceeds the degree of flattening in II. m. callodonte. The enamel 

 folds are complicated much as in H. gracile, H. richthofeni, and H. 

 mohavense. 



OEIGIN OF EICAEDO HIPPAEIONS 



The close similarity of the Ricardo hipparions to the types in 

 existence at approximately the same time in Asia and Europe is 

 probably not to be explained on any hypothesis other than that of 

 common origin. As the gap between Hipparion and the brachyodont 

 horses seems more clearly bridged in America than in Europe, there 

 is good reason for looking to America or some intermediate region 

 for the ancestors of this group. Among the species assembled in the 

 North American genus Merychippus there are a number of forms that 

 are very close to Hipparion, and that seem in their evolution to trend 

 directly toward that type. From available evidence it appears prob- 

 able that Hipparion is derived from the Merychippus group. 



With special reference to the origin of the Ricardo species it seems 

 worth noting that there is present in the Upper Miocene Barstow 

 fauna of the Barstow syncline an advanced form of Merychippus 

 differing but little from Hipparion. In the Barstow Merychippus the 

 crowns are a little shorter, they are more strongly curved, the 

 mesostyle is relatively heavier at the proximal end, the enamel folds 

 are less complex, and the cement is a little less abundant than in 

 II. mohavense. While it is improbable that the known Hipparion 

 forms of the Ricardo fauna are descended directly from any known 

 Hipparion-like species of Merychippus of the Barstow fauna, the 

 proximity of the two in morphologic characters, geographic situation, 

 and in time, strongly suggest close relationship. Very much of the 

 palaeontologic history of the Great Basin and Pacific Coast regions 

 is still unknown. The portions that are not yet known vastly exceed 

 the material available, and much of the Miocene record is still to be 

 discovered. In the present state of our knowledge we may consider 

 as reasonable the view that a line of descent may yet be traced from 

 a form near a Barstow Merychippus to a Ricardo Hipparion. 



