182 COPE. (Vou. III. 
the two crests of the ulna the articular surface of the radius is 
naturally concave, receiving the convexities of the humeral con- 
dyle. The external cotylus has become a groove corresponding 
to a crest of the humeral condyle, which has been developed by 
gradual modification of a strong convexity, such as is seen in 
Anoplotherium and Oreodon, and the Condylarthra (Plate I. 
Fig. D), and which is characteristic of many Unguiculata. Its 
keel or tongue-like character may be ascribed to the action of 
torsion combined with impact, as we find to have been the case 
with the distal metapodial articulations. It has been homologized 
with the intertrochlear crest of the Anthropomorpha, but it is 
not homologous with it. It is homologous with the external 
condyle or trochlea, while the crest of the Anthropomorpha is 
really intertrochlear. The condyle is developed extad of this 
keel, coextensively with the head of the radius. The develop- 
ment of these two extensions is necessarily coextensive, since 
they are mutually dependent on each other for their growth 
stimulus. Judging by the character of Hyracotherium ventico- 
Jum, the external extension commences with the head of the 
radius, since the external condyle of the humerus is much more 
retreating proximad than it is in the species of Equus. It has 
developed distad in the latter, genus, so as to bear nearly the 
same transverse line as the internal condyle (Plate I. Fig. E). 
The mechanical cause of the origin of this peculiar external 
roller of the distal humeral condyle is the same as that which 
has given origin to a similar structure on the external sides of 
the distal extremities of the metapodial bones. This is the ex- 
cessive use consequent on the very frequently externally diver- 
gent relation of the distal to the proximal element in the case; 
here, of the fore arm on the humerus. This occurs constantly 
in the most rapid movement of a diplarthrous Ungulate. On 
alighting on the ground at the end of a leap, the anterior foot 
supports the entire weight for a shorter or longer time accord- 
ing to the gait, the foot resting below the middle line of the 
body. According to Allen? the foot even crosses the middle 
line, and rests on the opposite side of that line to which it really 
belongs. Thus the remainder of the leg is abducted or bent 
1 Tertiary Vertebrata, Cope, Pl. XLIX. 4, Figs. 2, 3. 
2 Materials fox a Memoir on Animal Locomotion ; from the Report of the Muy- 
bridge work at the University of Pennsylvania, 1888, p. 58. 
