166 THE WONDERS OF GEOLOGY. Lect. IJ 



of the hands ; but which are not capable of being retracted 

 into a sheath, as in the feline tribes. The arms are double 

 the length of the legs, and, from the construction of the 

 limbs, the animal, when it walks, or rather crawls on the 

 ground, is obliged to drag itself along on its elbows. But 

 these creatures are destined to inhabit trees ; their proper 

 element is on the branches, and they can pass from bough 

 to bough, and from tree to tree, with a rapidity which soon 

 enables them to lose themselves in the depths of the forests. 

 They live on the leaves and the young shoots, and unless 

 disturbed, never quit a tree till they have stripped off 

 every leaf. To avoid the labour of a descent, they drop to 

 the ground, previously coiling themselves into a round ball, 

 in which state, while attached to the branch, they may be 

 taken alive. Thus the habits and economy of the sloth 

 point out the necessity for a peculiarity in the structure of 

 its claws. The monkey leaps and swings himself from tree 

 to tree, and catches at will the branches or the trunk ; but 

 the sloths do not grasp ; their claws are mere hooks to hang 

 by, and their great strength is in their arms. They never 

 unfix one set of hooks until they have caught a secure hold 

 with the other, thus hanging by their arms and legs, while 

 their bodies are pendant; and they sleep in the same po- 

 sition. The bones of the arm are constructed to suit these 

 conditions. The humerus has a long internal condyle for 

 the origin of large muscles to move the enormous claw^s ; and 

 there is an opening for the passage of the principal nerves 

 and blood-vessels, to protect them from the pressure to which 

 they would be exposed from the powerful muscular action : 

 and the radius (one of the bones of the fore-arm) is con- 

 structed to allow of a free rotatory motion to the limb. In 

 the extinct Sloths a similar conformation is maintained, 

 but somewhat modified to suit the different physical con- 

 ditions under which they existed. There are three genera 

 well established from the fossil remains: the names assigned 



