_Bradypus._[92]—Three-toed Sloths. Teeth usually ⁵⁄₄ on each side; no tooth projecting greatly beyond the others; the first in the upper jaw much smaller than any of the rest; the first in the lower jaw broad and compressed; the grinding surfaces of all much cupped. Vertebræ: C 9, D and L 20 (of which 15 to 17 bear ribs), S 6, C 11. All the known species present the remarkable peculiarity of possessing nine cervical vertebræ, _i.e._ nine vertebræ in front of the one which bears the first thoracic rib (or first rib connected with the sternum, and corresponding in its general relations with the first rib of other mammals); but the ninth, and sometimes the eighth, bears a pair of short movable ribs. The arms or fore limbs are considerably longer than the hind legs. The bones of the fore arm are complete, free, and capable of pronation and supination. The hand is long, very narrow, habitually curved, and terminates in three pointed curved claws, in close apposition with each other. The claws are, in fact, incapable of being divaricated, so that the hand is reduced to the condition of a triple hook, fit only for the function of suspension from the boughs of trees. The foot closely resembles the hand in its general structure and mode of use; the sole being habitually turned inwards, so that it cannot be applied to the ground in walking. The tongue is short and soft, and the stomach large and complex, bearing some resemblance to that of the ruminating Ungulates. The windpipe or trachea has the remarkable peculiarity among mammals—not unfrequent among birds and reptiles—of being folded on itself before it reaches the lungs. The mammæ are two, and pectoral in position.