54 



University of California Publications in Geology [Vol.9 



The West-American forms nearest P. tehonensis are a rare Prolo- 

 hippus or Pliohippus represented by no. 21423 from the Barstow 

 Upper Miocene, and a form referred to Pliohippus occurring in the 

 lower portion of the Jacalitos Pliocene of the North Coalinga region. 

 The Barstow species is near P. tehonensis in form and dimensions, 

 but differs slightly in form of the protocone, strength of the enamel, 

 and possibly in degree of cementation of the crowns. It seems to the 

 writer doubtful whether the Barstow and Tejon Hills forms are 

 specifically identical. The North Coalinga species described by Arnold 

 and Anderson 1 from the Jacalitos formation, and determined as Plio- 

 hippus, approaches P. tehonensis very closely and may be specifically 

 identical. As yet we have only three teeth from the Jacalitos and all 

 represent M 3 , so that a fully satisfactory comparison cannot be made. 



Pliohippus tantalus of the Ricardo Pliocene is larger and heavier 

 than the Tejon Hills species, its fossettes are larger and differently 

 formed. A Pliohippus species from Ricardo near P. mirabilis re- 

 sembles the Tejon Hills species in dimensions, but seems to differ in 

 details of form of fossettes and is presumably not of the same species. 



Measurements of Type Specimen, No. 21779 



?M', anteroposterior diameter 22.5 mm. 



?M\ transverse diameter 23.7 



!M', anteroposterior diameter of protocone « 8.4 



a, approximate. 



HIPPAEION MOHAVENSE CALLODONTE, n. var. 

 Type specimen, no. 21311, a finely preserved dentition including the upper 

 and lower cheek-teeth with several of the incisors. Collected in the Ricardo 

 Pliocene near Ricardo Postoffiee, on the western border of the Mohave Desert, 

 by Emerson M. Butterworth. 



In this form the dentition differs noticeably from that of the 

 typical Hipparion mohavense of the Ricardo Pliocene. The protocone 

 is more strongly flattened transversely, while the enamel pattern of the 

 teeth in general shows fewer deep, smoothly-rounded folds. The 

 plications of the enamel in this form tend to be more distinctly 

 angular, and there are more numerous minute folds. The dimensions 

 differ somewhat from those of the typical H. mohavense. The differ- 

 ences in dimensions and in pattern of the cheek-teeth separating this 

 variety from typical H. mohavense may be due in some part to varia- 

 tion in stage of wear, but the characters seem sufficiently marked to 



i Arnold, R., and Anderson, R., Bull. U. S. Geol. Survey, no. 398, p. 98, 1910. 



