“Ai” is the common name given in books to the Three-toed Sloths. They were all comprised by Linnæus under the species _Bradypus tridactylus_. More recently Dr. Gray described as many as eleven species, ranged in two genera, _Bradypus_ and _Arctopithecus_; but the distinctions which he assigned both to species and genera do not bear close examination. Some are covered uniformly with a gray or grayish-brown coat; others have a dark collar of elongated hairs around the shoulders (_B. torquatus_); some have the hair of the face very much shorter than that of the rest of the head and neck; and others have a remarkable-looking patch of soft short hair on the back between the shoulders, consisting, when best marked, of a median stripe of glossy black, bordered on each side by bright orange, yellow, or white. There are also structural differences in the skulls, as in the amount of inflation of the pterygoid bones, which indicate real differences of species; but the materials in our museums are not yet sufficient to correlate these with external characters and geographical distribution. The habits of all are apparently alike. They are natives of Guiana, Brazil, and Peru, and one if not two species (_B. infuscatus_ and _B. castaneiceps_) extend north of the Isthmus of Panama as far as Nicaragua. Of the former of these Dr. Seeman says that, though generally silent, a specimen in captivity uttered a shrill sound like a monkey when forcibly pulled away from the tree to which it was holding. _Cholœpus._[93]—Teeth ⁵⁄₄; the most anterior in both jaws separated by an interval from the others, very large, caniniform, wearing to a sharp, bevelled edge against the opposing tooth, the upper shutting in front of the lower when the mouth is closed (Fig. 59), unlike the true canines of heterodont mammals. Vertebræ: C 6 or 7, D 23-24, L 3, S 7-8, C 4-6. One species (_C. didactylus_) has the ordinary number of vertebræ in the neck; but an otherwise closely allied form (_C. hoffmanni_) has but six. The tail is very rudimentary. The hand generally resembles that of _Bradypus_; but there are only two functional digits with claws—those answering to the second and third of the typical pentadactylate manus. The structure of the hind limb generally resembles that of _Bradypus_, the appellation “two-toed” referring only to the anterior limb, for in the foot the three middle toes are functionally developed and of nearly equal size. _C. didactylus_, which has been longest known, is commonly called by the native name of _Unau_. It inhabits the forests of Brazil. _C. hoffmanni_ (Fig. 58) has a more northern geographical range, extending from Ecuador through Panama to Costa Rica. Its voice, which is seldom heard, is like the bleat of a sheep, and if the animal is seized it snorts violently. Both species are very variable in external coloration. [Illustration: FIG. 59.—Skull of Two-toed Sloth (_Cholœpus didactylus_). From _Proc. Zool. Soc._ 1871, p. 432.]