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 



Fig. 369. — Manus of Three- 

 Toed Sloth (Bradypus 

 tridactyhis). 



the first and fifth digits, 

 and the other three are long 

 and curved, each being 

 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 

 other digits being lost.] 

 The metacarpals and the 

 proximal phalanges are 

 fused together into one 

 bone, and with them are 

 joined the vestigial metacarpals of digits one and ?iYQ. 

 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 



Note the three long recurved claws, the 

 fusion of the first phalanges and the meta- 

 carpals into one bone, the fusion of sca- 

 phoid and trapezium and of trapezoid and 

 OS magnun. The unciform is round the 

 corner, making six carpal bones instead of 

 the usual eight. 



