MAMMALIA. 535 
a habitat demands it. In the cursorial types we have 
seen that this movement is given up, the bones being per- 
manently crossed or even fused. On the other hand, the 
arboreal habit, like the cursorial, does not entail differential 
use of the digits, and there is a corresponding reduction 
in their number and complexity. As in the cursorial 
types, it is the digits near the central axis that alone re- 
main. Our type has lost 
the first and fifth digits, | Fig. 369.—Manus or THREE- 
and the other three are long ToED SLoTH (Bradypus 
and curved, each being tridactylus). 
armed with a long curved 
claw. The digits are in- 
capable of independent 
motion and are largely 
enveloped in one fold of 
skin. In fact, the hand is 
reduced to the condition 
of a triple hook, fit only 
for the function of suspen- 
sion from the boughs of 
trees. [The two-toed sloth 
has, in addition, lost its 
fourth digit, and the tree 
anteater (Cycloturus) has 
gone a stage further, the 
third digit (cf horse) having 
a very large claw and the 
second a: smaller one, the Note the three long recurved claws, the 
other digits being lost. | fusion of the first phalanges and the meta- 
The metacarpals and the ‘mals io one one, the fasion of se: 
proximal phalanges are 0s magnun. The unciform is round the 
. : corner, making six carpal bones instead of 
fused together into one the usnaleight. 
bone, and with them are 
joined the vestigial metacarpals of digits one and five. 
The carpal bones are quite immovable, and the scaphoid 
is fused with the trapezium, as also is the os magnum with 
the trapezoid. 
This modification allows the sloth to hang from the 
boughs of trees without any muscular effort, and, indeed, 
it is said so to hang after death. -At the same time, it 
