180 COPE. [Vot. III. 
active use. It results, also, in the lower Mammalia that the hu- 
meral condyles project beyond the ulna and radius on each side. 
The torsion experienced by the limb at the moment of contact of 
the foot with the ground, produces a pressure of the inner end of 
the condyle forwards and outwards against the corresponding part 
of the ulna, and radius if it extend so far inwards. The external 
part of the humeral condyle is correspondingly pressed back- 
wards. The result is a flare of the inner anterior and posterior 
external edges of both the condyle and cotylus, if there be any 
free margin to either. Hence the obliquity of the ulnar cotylus 
in Mammalia, whose locomotion is on the ground. Those 
which, like the Anthropoid apes, swing themselves from branch 
to branch of trees, experience alternate torsions of the fore leg 
in both directions. Hence the edges of the ulnar cotylus are 
flared equally in both directions precisely as the strain on the 
material requires (Plate I. Fig. B). To this torsion we can as- 
cribe also the development of a flange on the inner anterior edge 
of the humeral condyle, in Condylarthra and Unguiculata in gen- 
eral, and the corresponding oblique truncation of the adjacent 
edge of the head of the radius, and the posterior flange of the 
cotylus of the ulna on the external side. The humerus forms no 
flange on the external side, because it does not overhang the ulna 
sufficiently ; the ulna being itself the external bone of the fore 
arm. Where the humerus does not overhang the inner side, as 
in Hyrax, there is no flange of the former, and no truncation 
of the head of the radius on the inner edge (Plate I. Fig. E). 
In the lines of the Unguiculates, of the Edentates, and of the 
Taxeopoda, there is an additional development of the power of 
the supination of the manus. This is accomplished by the rota- 
tion of the distal and of the radius round that of the ulna, and 
the rotation of the head of the radius on its own axis. So the 
head of the radius becomes more and more round, in consequence 
of the mechanical constriction of its long axis, till in the great 
anteater, the sloth, and the monkeys, it is perfectly round, and 
rotates in its ligamentous ring, while maintaining its position on 
the front of the ulna. In both of these types it lies on the 
external side of the middle line, owing to the habit of supination, 
which throws the radius more and more outwards. Perhaps it 
is for this reason also that the ulna extends inwards proximally, 
so as to permit of no internal humeral flange in both forms. 
