158 FOSSIL MAMMALIA. 
occur in no other animals except the Armadillo 
and Chlamyphorus ; both of which are continu- 
ally occupied in digging for their food. 
The articulation of the leg with the hind-foot 
is admirably contrived for supporting the enor- 
mous pressure of downward weight ; the astra- 
galus (z), or great bone of the instep, being nine 
inches broad, and nine inches high, is in due 
proportion to the lower extremity of the tibia, or 
leg bone, with which it articulates ; and rests 
upon a heel bone, of the extraordinary length of 
seventeen inches, with a circumference of twenty- 
eight inches. This enormous bone, pressing on 
the ground, gives a firm bearing and solid sup- 
port to the continuous accumulation of weight, 
which we have been tracing down from the 
pelvis through the thigh and leg : in fact the 
heel bone occupies nearly one-half of the entire 
length of the hind- foot ; the bones of the toes 
are all short, excepting the extreme joint, which 
forms an enormous claw-bone ; larger than the 
largest of those in the fore-foot, measuring 
thirteen inches in circumference, and having 
within its sheath a core, ten inches long, for the 
support of the horny claw with which it was 
invested. The chief use of this large claw was 
probably to keep the hind foot fixed steadily 
upon the ground.* 
* It is probable that the large thick claw, PL 5 5', was placed 
on the second toe of the hind-foot. Its size approaches nearly 
to that of the first toe of this foot, and both of these differ mate- 
