1919] Merriam: Tertiary Mammalian Faunas of Mohave Desert 479 



Some significance may attach to the fact that this form, having a 

 certain resemblance to Parahippus, but being relatively primitive, 

 occurs in strata which were presumably deposited in a later period 

 than the time of maximum development of the genus Parahippus. On 

 the other hand, the Barstow form, being somewhat more advanced 

 than Archaeohippus in most respects, and occurring in strata pre- 

 sumably younger, might be considered a product of modification from 

 Archaeohippus. It is interesting to note that in the development of 

 the crochet, in which one would expect advance, the Mohave form is 

 more primitive than the Middle Miocene Archaeohippus. 



The form represented by Parahippus? mourningi might be assigned 

 tentatively to a place with Archaeohippus, as an advanced stage with 

 protoconule and hypostyle more progressive, cingulum of the proto- 

 cone side absent and complication of the metaloph not more advanced. 

 It might be referred to Parahippus, as a primitive stage with crochet 

 undeveloped, though the metaloph shows secondary folding, and with 

 metaconid and metastylid in beginning separation. Its reference to 

 one of the described genera depends somewhat upon the extent to 

 which the limits of these groups may be expanded by later studies. A 

 reference to Parahippus is apparently open to fewer definite objec- 

 tions than a reference to Archaeohippus. 



MEEYCHIPPUS Leidy 



The remains of Merychippus are much the most common fossils 

 throughout the Barstow wherever it has been examined. In the 

 uppermost or Fossiliferous Tuff member of the Barstow syncline, 

 mammalian remains are relatively much more abundant than in the 

 other portions of the beds and Merychippxis is the most common form. 

 One zone, in which bones of this form were especially abundant was 

 known in the field as the Merychippus bed. According to Baker, 

 this stratum could be traced for a considerable distance. The pre- 

 sumption is that the Merychippus bed represents a deposit formed 

 with unusual slowness, and possibly at a time when conditions 

 bringing retardation of accumulation made the region an especially 

 favorable habitat for animal life. 



The Merychippus forms found in the Barstow seem to differ from 

 the species of the Mascall and Virgin Valley, and also from all of the 

 species in the Ricardo fauna. The Barstow forms are all more 

 progressive than those of the Mascall stage, and with one possible 



