270 Origin of the Foot Structures of the Ungulates.  [,April, 
powers of running, the reduction is seen first in the posterior feet, 
which propel the body much more than the fore feet, This view 
is well illustrated in the Perissodactyle families, the masonry of 
which have the digital formula 4—3. 
No reason has ever been suggested, so far as I am aware, in 
explanation of the fact that one series of ungulates has retained 
two digits, and the other only one; that is, why there should 
have been two kinds of digital reduction instead of one kind. 
In seeking for an explanation, we will remember that the tarsus in 
the odd or single-toed line, is bound together by fixed articula- 
FIG, 2. 
Fic Right posterior foot of a = geass ait es from New Mexico, one 
ss oa size. From Report Expl. W. . M. Wheeler, Iv. TL L 
Right posterior ve of yaya mega Cope, from Colorado, one- 
half: macaeel size. From Report U, S. Geol. Surv. Ter V. Hayden, 1v, Pl. CXXX. 
tions, while in the cloven footed line it is interrupted by the hinge 
between the first (astragalus), and second rows of bones. The 
hinge-joint being more liable to luxation than the fixed articula- 
tion, requires a wider basis of support, such as would be furnished 
by two divergent digits, rather than by a single central one. 
In the early types, where the median digits are slender, the 
mechanical advantage in favor of the bidigital over the unidigital 
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 
