502 University of California Publications in Geology [Vol- H 



?ual phalanges of digit III of even the smallest form (fig. 

 Barstow fauna are relatively somewhat wider than in one 

 perfect specimen of a Merychippus-like horse from 

 the Mascall available for comparison, but have nearly 

 the same size and proportions as another Mascall 

 specimen. The difference between the smallest and 

 largest forms in the Barstow collection is mainly one 

 of size. In all of the specimens the anterior end is 

 subacute, with the terminal cleft well marked. The 

 postero-lateral wings project slightly behind the 

 inferior margin of the proximal articular face. 



Compared with casts of the feet of Protohippus 

 sejunctus from the American Museum, the largest 

 ungual phalanges in the Barstow collections have 

 nearly the same size, but seem to show slightly 

 smaller lateral wings, and are possibly a little higher 

 posteriorly. 



Measurements of Metapodial III 



Fig. 69. 



Merychippus, sp. 

 Third metatarsal, 

 no. 21407, X Vj. 

 Barstow Miocene, 

 Mohave Desert, 

 California. 

 Fig. 70. 



Merychippus, sp. 

 Distal end of 

 third metatarsal 

 with first phalanx, 

 no. 21468, X V- 2 - 

 Barstow Miocene, 

 Mohave Desert, 

 California. 



Anterior, no. 22372 

 Posterior, no. 19817 

 Posterior, no. 23130 



165.2 mm. 



180.5 



183 



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16 



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 24.2 

 25 



Metapodial III of the Merychippus species of the 

 Barstow fauna varies considerably in size, but the 

 general outlines of the shaft do not vary greatly in 

 either front or hind limb. The distal keel is well 

 developed in all specimens. In some individuals the 

 keel is scarcely visible on the anterior side of the 

 distal articular face of even the posterior metapodial 

 III, but careful examination will show a faint ridge 

 reaching the proximal end of the anterior face. On 

 some of the large specimens, as in no. 21269, the 

 keel is strongly marked up to the proximal end of 

 the anterior side of the distal articulation. 



In metacarpal III the angle between the magnum 

 and unciform facets ranges from near 112° to a 



