No. 2.] THE HARD PARTS OF THE MAMMALIA. 17I 
the metapodials. Hence the apparent outward movement of 
the latter. The structure, however, represents an inward pres- 
sure of the second row of carpal and tarsal bones on them. The 
bones of the second row are then subject in locomotion to two 
influences which tend to shift them from their position. First, 
the pressure outwards on plantation, derived from the weight 
borne by the leg, and resisted by the metapodials on the ground; 
and second, pressure inwards of the weight, on its transfer to 
the inner side of the foot, also resisted by the metapodials on 
the ground on recovery. The weight is the same in both cases, 
and there is no reason to doubt the equal velocity of the two 
strains. Hence the effect of the one is neutralized by the 
other, and the bones of the second row retain their position.! 
It is obvious that in the beginning of the rotation of the first 
on the second row of carpals and tarsals, of a taxeopodous 
mammal (Fig. 16), the inferior bounding angles of the former 
would be arrested by the superior bounding angles of the latter, 
especially if there should be any play of longitudinal motion of 
the digits and carpal and tarsal bones resting on them. Such 
play there would be in a taxeopodous foot. This arrest would 
flatten the opposing angles and produce planes or facets, and 
diplarthrism would have its beginning. The angles separating 
these planes would penetrate deeper into the fissures separating 
the bones under the influence of concussions. (See carpus of 
Boéchoerus, Fig. 15.) In some instances, as in the Oreodon- 
tide, some Tragulidz, and Elotherium, the lunar has pene- 
trated so far as to almost divide the second row of carpals. 
An important result of the outward movement of the bones 
of the first carpal and tarsal rows has been to leave unsupported 
the internal element of the second row, the trapezium and the 
entocuneiform. As a consequence of this, these elements come 
to have a mere lateral attachment, which is not favorable to the 
rigidity of the pollex and hallux which are attached to them. 
This condition leads to disuse, which the strain of the inner 
side of the foot in recover is@not sufficient to overcome. The 
external digit of each foot, on the contrary, remains, since its 
1Jn my original discussion of diplarthrism (American Naturalist, Dec., 1888), 
I omitted to consider the history of the second row of carpals and tarsals. This de- 
ficiency is now supplied. I have been guided to the explanation by Dr. H. Allen’s 
obseryations on p. 50 of his exposition of the Muybridge photographs, 
