No.2.] THE HARD PARTS OF THE MAMMALIA. 165 
expansion of the external border of the head of the radius over 
the adjacent external part of the ulna. The latter forms, in the 
Diplarthra, a more or less evident ridge, corresponding with the 
external condyle of the humerus, while the internal border forms 
another ridge, which corresponds to the internal condyle of the 
humerus. These elevations are due to the existence of greater 
friction, z.¢. stimulation, in front of convex condyles than is pos- 
sible at the point of the cotylus which receives the groove of 
the condyles. A groove of the coronoid surface of the ulna, 
opposite the groove of the condyles, exists accordingly in most 
ungulates, since here contact, friction, and impact are at a mini- 
mum. The head of the radius fits this groove, which is at first 
quite shallow, as in the peccary and hog. In the extension of the 
head of the radius externally, the latter naturally embraces the 
external ridge of the coronoid surface of the ulna. This embrace, 
together with the opposite descent of the head of the radius into 
the median groove, constitute a very strong double open peg 
and notch, or short tongue and groove, articulation. This is 
most pronounced in the highest Diplarthra, as the crests of the 
ulna tend to become more elevated under the influence of the 
impacts and friction conveyed to them through the convex parts 
of the humeral condyles. This articulation, like all those of the 
Diplarthra, is a protection against dislocation from lateral strains 
and blows. For an explanation of the origin of the elbow joint, 
see page 179 (Plate I.). 
B. The Carpus and Tarsus. 
The conversion of a taxeopod into a diplarthrous ungulate 
has been accomplished by the rotation outwards of the lower 
leg with the first row of the carpus and tarsus, on the second 
row ; or else by the rotation inwards of the second row on the 
first, in both the fore and hind feet. The question to be solved 
here is, which row has moved from its primitive relation, or in 
what direction has the energy of rotation been applied. After 
long and careful observation of the locomotion of living Mam- 
malia, and especially Diplarthra, I have reached the following 
definite conclusions : — 
In locomotion each foot occupies two relations to the act. 
The first is the setting-down of the foot, or plantation; the 
other is the lifting of the foot from the ground, or vecovery, to 
