1893.] Evolution of Metapodial Keels of Diplarthra. 429 
metapodials, which would throw the keels so far back as to 
seldom come in contact with the proximal phalanx. In the 
middle toe or third digit, of the digitigrade foot however, the 
main flexure of this toe is momentarily transferred from the 
articulation between the metapodial and first phalanx to that 
between the second and third phalanges in the act of walking, 
and this straightening out of the first two phalanges, so as to 
bring them in a line with the long axis of the metapodial, has 
caused the keel to impinge upon the lower edge of the articu- 
lar surface of the first phalanx which has become distinctly 
notched. 
FIGURE 3.—Mesohifpus. Outer view of the left hind foot. B, Distal surface of 
median metatarsal, C, Proximal surface of median first phalanx. (Collection 
Amer. Mus. Nat. Hist.) 
It follows that in such forms as the tapir, rhinoceros, Hyra- — 
cotherium, and in fact in all of the truly digitigrade Perisso- 
dactyla, the first phalanx of the median digit is always 
ae 
