Several kinds of three-clawed Sloths have been called Ai; for instance, the Yellow-throated Ai, and De Blainville’s Ai, and all have been named _Bradypus tridactylus_. Dr. Gray, however, satisfied himself that the kind which was first described by Cuvier as the Ai, and which is figured in Prince Maximilian of Neuwied’s “Animals of Brazil,” is the same as one which has since been called _Arctopithecus Ai_, or _Arctopithecus flaccidus_. The word _Ai_ is taken from the noise made by the animal, and the term _flaccidus_ relates to its long hair. The true Ai inhabits Venezuela and Peru, and has very long flaccid grey hair mottled with white. There is an abundant under-fur of a blackish-brown colour, which has white and black in spots and blotches. There is a small spot between the shoulders on the back, where the fur is soft and woolly, and a broad, short, blackish streak there, with a white or orange ring around it. The claws are coloured brown. The head has a curiously-cut short and turned-up nose appearance, and is furnished with coarse shaggy hair, disposed on the crown in a diverging manner. The short hair of the face contrasts with the long, shaggy, shrivelled, dry, hay-looking hair of the body. This hair is coarse and flattened at the ends, but it is exceptionally fine at the roots, and it greatly resembles in colour and texture some of the vegetation of the trees on which it lives. The eyes are bright, and are surrounded by a dark ring. Several species of the genus Arctopithecus have been described which live in Guiana, Bolivia, Brazil, Peru, and Venezuela. [Illustration: SKULL OF AI.] The next genus of the Sloths is represented by THE TWO-FINGERED SLOTH (THE UNAU).[60] There are several kinds of Sloths with two “toes,” or rather with two fingers ending in claws on the fore limb, but the differences between the species are not very readily appreciated. They are differences which can be recognised, but it is doubtful whether the possession of dark brown or pale brown hair is sufficient to decide that there are more than one species. The common Unau Sloth is usually of a darker tint than the others, but there is no doubt that the specimens in museums of all these Sloths vary much in the colour and length of the hair. Thus the hair may be generally dark, and the hairs of the crest on the back of the head may be white, and more or less tinted with bright green; or the hair may be short, of a dark brown colour, paler on the rump, much paler on the head, cheeks, and chin; a band may be across the nose, and the orbits dark brown. Others of the same species have very long hair, of an uniform dark tint, paler on the head and redder beneath, whilst one from Juan de Fuca has short hair, without any indication of a crest. From Brazil there are specimens with long paler hair and a crest. All these specimens, however, have pale whitish claws.