178 COPE. [Vou. III. 
do not admit of flexion and extension of the elements thus con- 
nected. A transition from a ginglymoid to a fixed articulation 
is seen in the modification of a Condylarthrous convex distal 
articular surface of the astragalus into the fixed or facetted 
distal articular surface of the Perissodactyla (Figs. 21-22c). 
Another modification of the Condylar- : 
throus astragalar surface is that seen in 
the Artiodactyla. Here the mobility 
continues, but it is divided into two con- 
dylar faces which necessarily permit 
motion in one direction only. It ap- 
pears easy to account for the modifica- 
tions of the Condylarthrous astragalus 
into that of the two orders mentioned, 
by supposing that it has 
been due to long-continued 
impacts applied at right 
angles to the surfaces con- 
cerned ; that is, with the 
length of the foot. Ina 
plantigrade animal such 
impact is either transverse 
or oblique to the length of 
the foot, and as digitigrad- 
ism progresses, the direc- 
tion of impact is more and 
more in the long axis. In 
Fic. 22. 
Figure 21. — Phenacodus primevus, carpus and tarsus; @, carpus, proximal view, 
first row; 4, do. second row; ¢, tarsus, distal view of astragalus and calcaneum; @, prox- 
imal view of second row of tarsals. 
Figure 22.— Hyracotherium venticolum, posterior foot: a, left side; 4, front; 
¢, distal view of astragalus and calcaneum. 
Mammalia with soft pads on the feet, this chance does not oc- 
cur; as Carnivora, Proboscidia, etc. 
The movable or ginglymoid joints of the limbs are either sim- 
ple or trochlear. The former are the proximal extremities of the 
humerus and femur, and the distal extremity of the astragalus 
in all except Diplarthra. The trochlear are enumerated further 
on. That they all originated from the simple ginglymi is shown 
