198 COPE. [VoL. III. 
pressure on their sides, as in the case of the distal keels of the 
metapodials. 
The relative character of the phalanges of the different feet 
seen in Priodontes maximus finds a parallel in the Glyptodontide. 
Here the tongue-and-groove articulations are strong on the an- 
terior feet, and altogether wanting on the posterior! The 
ungues on the former are half-claws, and are evidently usable 
for digging, while those of the pes are hoofs, usable only for 
support or pressure. The relation of cause and effect is as 
obvious here as in the case of the Przodontes. 
Finally, the mechanical effect of the torsion strain on the 
convexity of a condyle in converting it into a keel, may be 
noticed in greater detail. The result mentioned appears in the 
distal condyles of the humerus in the Diplarthra, at the distal 
extremities of the metapodials in many Mammalia, and in the 
phalangeal articulations of fossorial types. The effect of a 
twisting motion of a convex or a concave surface when in close 
apposition, is to cause pressure and friction on each side of a 
central dead-point, which is at the centre of the curve. If these 
surfaces change their relative positions in one plane through 
flexion and extension, a line of dead points at the apex of the 
convexity is free from the pressure and friction which are devel- 
oped on each side of it. The greater the arc of movement in 
this plane, the more extensive will be this dead line. The great- 
est movement in arc results where the one surface has a shorter 
arc than the other. The concave surface has the smaller arc in 
the three positions named, and has also the greatest movement 
in arc, as it is in each case the distal element. 
The internal and external sides of the distal metapodial con- 
dyles are not similar; a character very distinct in the Artiodac- 
tyla (Fig. 42). This is simply due to the unequal pressure 
exerted on the two extremities of the condyle by the phalanges, 
owing to the divergent direction of the digits when serving as 
a support. In the distal end of the humerus the same effect is 
seen, the external part of the condyle nearly resembling the cor- 
responding part of the metapodial bones. This is traceable to 
the same cause, viz.: the divergent position assumed by the 
fore arm on the humerus, when the weight is supported on one 
1 See Burmeister, Archiv. fiir Anatomie u. Physiologie, 1865, p. 317, Pl. VIII. 
