500 University of California Publications in Geology [Vol. 11 



The mandible is not shown complete in any specimen. In no. 20029 

 (figs. 57a and 576), a young individual with not yet in function, 

 the horizontal ramus is seen to be much higher than in a young speci- 

 men of Merychippus isonesus from the Mascall Miocene. The sym- 

 physial region is not greatly widened, and the very small canine is 

 close behind I 3 . 



A lower jaw (fig. 47) associated with the skull no. 20039 does not 

 show the outlines of the ramus. In no. 21228 the form of the mandible 

 in the adult is seen to be much as in the young specimen represented 

 in no. 20029 (fig. 57a). 



Limbs. — Large numbers of skeletal parts found in the beds of the 

 Barstow syncline represent a Merychippus species corresponding 

 approximately to M. calamarius in stage of development and in size. 



The range of variation in the material extends from elements 

 representing small forms near the size of specimens of Merychippus 

 isonesus of the Mascall up to elements from individuals evidently 

 about double the size of the smaller type. In the absence of good 

 associated skeletal material it is not possible to make a thoroughly 

 satisfactory determination of the systematic position of the species 

 represented by the skeletal elements, but it is reasonable to assume 

 that the smallest represent Merychippus sumani as known from the 

 dentition, and that the largest specimens are from the M. intermon- 

 tanus type. 



In general the type of foot structure is similar to that of the 

 genus Merychippus as seen in M. isonesus. The metapodials are 

 slender, the lateral digits small and scarcely functional. The hoofs 

 of digit III are narrow, and with a marked terminal cleft. The narrow 

 astragalus has a deep subacute trochlear groove. 



The abundant proximal phalanges of digit III (figs. 66 and 67) 

 are much more slender than those of Hypohippus (fig. 29) and show 

 a marked median constriction. There is a suggestion of less flattening 

 and more distinct rounding of the shaft than in a number of specimens 

 presumably representing Merychippus isonesus from the Virgin Val- 

 ley Miocene. The groove for the trochlear ridge of the metapodials is 

 commonly not well marked at the anterior side of the proximal articu- 

 lar face; but in a few of the largest specimens (fig. 68) it is strongly 

 marked on the anterior side corresponding to the development of the 

 metapodial keel. 



The second phalanges of digit III are easily distinguished by their 

 narrower form from the wide, relatively short phalanges of Hypo- 

 hippus. 



