GASSELL'S POPULAR NATURAL HISTORY. 



its .structure, and of great breadth, separating very widely the thigh-bones ; these parts, too, are 

 directed obliquely outwards, while the limb, from the knee downwards, has an inward inclination. 

 The wrist and ankle are so constructed, that the palm, or sole, instead of being directed to the surface 

 of the ground, as in other animals, is turned inwards towards the body, in such a manner as to make 

 it impossible for the sloth to place the sole of its foot straight down on a level surface, and it is com- 

 pelled, under such circumstances, to rest on the external edge of the foot. The hip-joint, as in the orang- 

 utan, is destitute of the usual ligament, * that the head of the thigh-bone may have greater freedom 

 of motion. In the Ai, or Three-toed Sloth, the neck consists of nine vertebra;, instead of seven, which 

 is' the usual number in the mammalia. Moreover, the two tables of the sknll, in all the species, are 

 separated by lafge air-cells, so that the small, bird-like brain is actually defended by a double case, so 

 that, should the branch give way to which the sloth is clinging, it may be defended from injury. 



Professor Owen remarks: "The sloths illustrate the affinity, or tendency, to the oviparous 



type, by the supernumerary cervical vertebras, 

 supporting false ribs, and by the convolution 

 of the wind-pipe in the thorax, in the three- 

 toed species ; by the lacertine, or lizard-like 

 character of three-and-twenty pairs of ribs in 

 the Uiiau ; and by the low, cerebral develop- 

 ment, by the great tenacity of life, and long- 

 enduring irritability of muscular fibre in both 

 species." The force of the muscles is scarcely 

 credible, and perfectly harmonises with the 

 skeleton, so entirely adapted to a life in trees. 

 The long arms of the .sloth, provided with 

 strong hooks, which are drawn to the palm, 

 just as it is with the claws of the hinder 

 feet, by means of elastic ligaments, enable it 

 to reach a distant branch, and there to fix 

 itself with facility ; or, while clinging to one 

 branch, it can draw towards itself another 

 loaded with leaves ; buds, or fruits, on which 

 it can banquet at pleasure. Even its rigid 

 paw can be used as a hand, and made to convey 

 its food to its mouth with great address. 



The sloths have no incisors, but canines 

 and molars only ; and in the Ai the canines 

 are diminutive, and in all respects veiy- 

 similar to the other teeth. There are 

 eight molars in the upper jaw, and six 



in the lower, four and three on either side respectively. They are cylindrical, unrooted, con- 

 sisting, as Owen has shown, " of a centre of vascular dentine, or ivory, the whole enveloped by a layer 

 of cceinentAim, characterised by numerous minute calciferous cells." Ill fitted for grinding the food, 

 the teeth merely bruise it, or break down the tender leaves and buds ; their deficiency being probably 

 compensated for by a singular complication of the stomach, which is sacculated. The Ai is three-toed. 

 The head of the Ai is short, the face small and round, the ears concealed in the long hair which 

 surrounds them, the eyes small, and deeply sunk in the head, and the tail a mere rudiment. The 

 Indians like its flesh, and constantly pursue it. Of the Ai, naturalists reckon two distinct species, and 

 three or four varieties. 



THE COLLARED SLOTH.f 



THIS is a very distinct species, even in the bony structure of the skull. Its face is naked and of a black 

 colour ; the hair of its body less flattened :ind withered-looking than is the case in the -common species ; 



i urn teres. t Bradypus collaris. 



THE UNAU, OP. TWO-TOED SLOT1F. 



