158 
FOSSIL MAMMALIA. 
occur in no other animals except the Armadillo 
and Chlamyphorus ; both of Avhich are con- 
tinually 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 hone of the instep, being nine 
inches broad, and nine inches high, is in du® 
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 ot 
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 tho 
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 tb^ 
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, PI. 5 5', was placed 
on the second toe of the hind foot. Its size approaches near!} 
to that of the first toe of this foot, and both of these differ mate- 
