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232 containing not more than three or four known species, are limited to the tropical regions of South America.

The characters already enumerated as belonging to the Family are more strongly marked in this genus than in the Unau (Cholœpus). It is also distinguished by having three claws on the forefoot or hand: the thumb and little toe barely visible as rudimentary bones in the skeleton, being quite concealed by the skin. It possesses a tail, which, however, is short. The neck is lengthened, and endowed with a more complete power of rotation by the addition to it (in effect) of two bones&#59; which, however, by their possession of rudimentary ribs, are proved to belong to the back&#59; leaving the number of ¢rue neck-bones seven, as in all other Mammalia.

The Common Ai (Bradypus tridactylus, has a short round head, clothed with long rough hair, which diverges from the crown, like the human hair. The body is covered with long hair, coarse and flattened at the extremity, but as fine at the root as a spider’s thread. Its colours are light and dark brown, dispersed in irregular patches, and varying much in individuals&#59; an oval patch of finer shorter hair, of an orange-colour, is situated between the shoulders&#59; the face, throat, and breast are of a pale straw-yellow. It is about as large as a Cat. The expression of its countenance is very melancholy, as is the sound of its voice, which resembles the syllables aï, feebly uttered in a plaintive tone.