hairs around the shoulders (_B. torquatus_); some have the hair of the face 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. The habits of all are apparently alike. They are natives of Guiana, Brazil and Peru, and two species (_B. infuscatus_ and _B. castaneiceps_) extend north of the Isthmus of Panama as far as Nicaragua. Of the former of these a specimen in captivity uttered a shrill sound like a monkey when forcibly pulled away from the tree to which it was holding. In the species of unau, or two-toed sloths, _Choloepus_, the front tooth in both jaws is separated by an interval from the others, and is large and caniniform, wearing to a sharp bevelled edge against the opposing tooth, the upper shutting in front of the lower when the mouth is closed, unlike true canines. Vertebrae: C 6 or 7, D 23-24, L 3, S 7-8, Ca 4-6. One species (_C. didactylus_) has the ordinary number of vertebrae in the neck; but an otherwise closely allied form (_C. hoffmanni_) has but six. The tail is very rudimentary. The fore-feet generally resemble those of _Bradypus_, but there are only two functional digits, with claws; these answering to the second and third of the typical five-toed limb. 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. The premaxilla is well developed, and firmly attached to the maxilla; and there is always a perforation, or foramen, on the inner side of the lower end of the humerus. _C. didactylus_, which has been longest known, and is commonly called by the native name of unau, inhabits the forests of Brazil. _C. hoffmanni_ 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 (see SLOTH).