492 University of California Publications in Geology [Vol. 11 



that this specimen represents that species. The teeth in this specimen 

 differ from those of certain forms referred to Pliohippus in the 

 Ricardo fauna in the more clearly separate protocone and slightly 

 less cementation. 



A complete series of lower milk molars (no. 20029, figs. 57a and 

 57&) represents a relatively small form of the Mcrychippus group. 

 The crowns are about as advanced as in M. isonesus of the Mascall, 

 but are more heavily cemented. Dm^ is a very small peg-like tooth. 

 M 1 is a long tooth. This specimen represents either M. sumani or the 

 M. c. stylodontus form. The incisors and canines are missing from 



Figs. 57a and 57b. Merychippus sumani? Merriam. Mandible with milk 

 dentition, no. 20029, X %. Fig. 57a, lateral view; fig. 57b, dorsal view. Barstow 

 Miocene, Mohave Desert, California. 



this specimen. The alveolus for the inferior canine is very small, 

 and is situated close behind I 3 . 



A lower jaw with milk teeth somewhat larger than in no. 20029 

 was associated with the upper milk dentition of no. 21460, and prob- 

 ably represents the same specimen. The crowns are high and well 

 cemented, much as in Protohippus. This specimen presumably rep- 

 resents Merychippus intermontanus. 



In all of the lower milk teeth of the Mcrychippus forms of the 

 Barstow fauna the anteroexternal fold on the protoconid, and the 

 external tubercle between protoconid and hypoconid tend to develop. 



Comparison with a fine specimen, evidently representing Mery- 

 chippus isonesus (no. 1678, figs. 58 and 59) from the Mascall Miocene 

 in Oregon shows that the difference between the milk molars of the 

 Mascall and Barstow forms is not large. The crowns of the Barstow 

 species are possibly a little longer and the cementation somewhat 



