544 On the Effect of Impacts and Strains {July, 
cause the distal emargination of the astragalus in the Artiodac- 
tla, The proximal ridges of the phalanges are very weak, and 
the concavities in the extremity of the radius cannot be called 
trochlear, as they are adaptations to the carpal bones. 
I. The reduction in the number of toes is supposed to be due to 
the elongation of those which, slightly exceeded the others in 
length, in consequence of the greater number of strains and im- 
pacts received by them in rapid progression, and the complement- 
ary loss of material available for the growth of the smaller ones.: 
This is rendered probable from the fact that the types with 
reduced digits are dwellers on dry land in both orders, and those 
that have more numerous digits are inhabitants of swamps and 
ry it is supposed that the Perissodactyles 
FIG. 3. Fic. 4. 
Fic. 3.—Right posterior foot of Protohippus sejunctus Cope from Colorado, about 
one-half natural size. From Report U. S. Geol. Surv. Terrs., F. V. Hayden, Iv, 
Fic. 4.—Right posterior foot of Poébrotherium labiatum Cope, from Colorado 
three-fifths nat. size. From Hayden’s Report, Iv, Pl. cxv. 
