417 
foot to bend on the leg, there exists a conical pit in the top of the 
astragalus, in which the extremity of the fibula is inserted, like a pivot, 
the foot turning round like a vane on its staff*. Added to this, wdien 
the leg is vertical, the foot is nearly in a similar direction, standing on 
its edge, so that the animal cannot place the sole of the foot on the 
ground, but by stretching out the leg until it has placed it in almost a 
horizontal direction. 
The toes of the animal are inclosed, quite to the nails, in a stiff skin, 
which will allow only of their being bent and straightened all toge- 
ther. And to add to its difficulty in motion, several bones, which, in 
other animals, are always distinct, are here joined together. Thus 
the first phalanges of all the feet are united to the bones of the meta- 
carpus and metatarsus. In this manner one bone fills the place of 
eleven, or even of seventeen. 
The nails are of a monstrous length, and are weapons which, by 
enabling the animal to defend himself with considerable success, may 
be regarded as the only compensation for the disadvantages of the 
rest of its organization. But these animals, unable to draw back their 
nails, as the cats do, are obliged to curve them underneath, when they 
do not use them, and thus place their convex surfaces downwards. 
As in cats, so in the sloths, each claw is set, and retained in a bony 
glove -like sheath ; but in the cats the upper part of this sheath is most 
advanced ; whilst in the sloths, the lowest part is most forward. 
In the A'i, different from all other quadrupeds, are nine cervical 
vertebras ; an extraordinary singularity, characteristic of this particular 
species, and not an accidental or monstrous formation. Thus in the 
same genus exists a most essential difference of structure. 
The sloths, different from other animals, have no incisors. In the 
A'i, those teeth which might be regarded as the canine teeth, are not 
pointed, but are rubbed down obliquely : the upper ones backwards, 
and the lower ones at the sides. The teeth are most simple in their 
construction ; being a cylinder of bone, surrounded by an envelope of 
vol. hi. 3 H 
