426 The American Naturalist. [May, 
the didactyle foot may possibly be the correct one, but I am 
inclined to doubt it seriously. It certainly cannot apply to 
the monodactyle foot since it is inconceivable that any 
“pinching” could occur in flexion and extension of the 
phalanges upon the metapodials; for the reason that there is 
no spreading such as he describes in the didactyle foot. It is 
moreover manifestly impossible that the sesamoids could have 
come into play to form the keel, because the amount of exten- 
sion necessary to bring them into the proper position would 
be so great as to cause complete dislocation of the phalanx. 
How then can this extension of the keels be explained upon 
the basis of mechanics? 
FIGURE 1.—Coryphodon, Median digit of the Manus, posterior view. (Coll. Am. 
ist 
In the first place it is necessary to call attention to the fact 
that at least three positions have been assumed in the various 
stages of the evolution of the ungulate foot, viz: a planti- 
grade or subplantigrade position, in which not only 
the phalanges but the metapodials are applied to the ground 
in walking, as in the hind feet of Coryphodon and all the earlier 
forms; second, a digitigrade position, in which only the 
phalanges are applied to the ground in walking, as in such 
forms as the tapir, rhinoceros and the earlier representatives 
of the horse series ; and finally the unguligrade position 
in which the weight of the body is supported entirely upon the 
terminal or ungual phalanges. This position of the foot 
