ORIGIN OF FOOT STRUCTUPwES OF THE UNGULATES. 369 



tion, requires a, wider basis of support, such as would be furnished 

 by two divergent digits, rather than b}^ a single central one. 



In the early types, where the median digits are slender, the 

 mechanical advantage in favor of the bidigital over the undigital 

 arrangement is much more obvious than in modern genera. Late 

 in time, the horse developed the middle digit to such a width as 

 to form almost as good a support as the bidigital structure. In 

 the Eocene genera, the slender median digit could not have sus- 

 tained the weight on a hinge, without great risk of dislocation. 



Fig. to. 



Fig. 71. 



Fig. 70.— Eight posterior foot of a species of Corypliodon from New Mexico, one 

 half natural size. From Eeport Expl. W. of 100th Mer., G. M. Wheeler, iv, PI. lix. 



Fig. 71.— Right posterior foot of the rhinoceros, Aphelops megalodus Cope, from 

 Colorado, one half natural size. From Keport U. S. Geol, Surv. Terrs. F. V. Hayden 

 iv, PI. cx.\x. 



This explanation, it can be said, applies only to the posterior foot. 

 The posterior foot has, however, led the way in the evolution of 

 Ungulata, and the fore foot may have followed in accordance 

 with the law of antero-posterior symmetry in growth. A curi- 

 ously malformed deer from Mendocino County, Cal., throws some 

 light on this subject. It has apparently a single functional digit 

 on each foot. Examination shows that the posterior foot is bi- 

 digital, but that the phalanges are fused ; while the anterior foot 

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