The dental system of the Sloths is the most simple that can well be conceived. They have no incisor teeth, but canines and molars only ; and in the B. tridaetylus the canines are diminutive, and in all respects very similar to the other teeth. The molar teeth are universally eight in the upper jaw and six in the lower, four and three on either side respectively. Their construction is most simple, consisting merely of a cylinder of bone, enveloped in enamel, and hollow at both ends,—at the upper by continual detrition, and at the under by default of ossifi cation. They have no lamina of enamel penetrating the body of the tooth, as in other herbivorous animals, which renders them such effective instruments in grinding and masticating vegetable substances. Hence it results, that the mastication of the Sloth must be extremely imperfect, though the defect of dentition is probably compensated in some degree by the superior complication of the stomach.
The genera Bradypue and Cholerpus, together with the extinct fossil animals which have been called Megatherium and Megalonyx, and which, with the form and organisation of a sloth, nearly equalled the elephant in size, constitute the Cuvierian family Tardigrada. Besides the difference of the canine teeth, which are completely developed in Cholmpue and in Bradypue, of the same form and subject to the same dctrition as the molars, these two genera are distinguished from one another by the number of toes on the fore feet, which are three in the Bradypue and only two in the Cieshrpes ; by the comparative length of the fore-arms, which are much longer in the former than in the latter ; by the number of cervical vertebne in the Bradypuo, as already mentioned ; by the equally unusual number of ribs in Cholxpue, which amount to no fewer than forty-six, the grestest number hitherto found in any mammal, the species of Bradypue having but thirty-two ; and by numerous other modifications which it is unnecessary to enumerate.
The Sloths are known to bring forth and suckle their young like ordinary quadrupeds. For this purpose they have two tnammse, which aro situated on the breast ; and the young Sloth, from the moment of its birth, adheres to the body of its parent till it acquires sufficient size and strength to shift for itself. The head of the Sloth or Ai (B. tridaetylns) is short, the face small and round like that of the American monkeys, the ears concealed in the long hair which sur rounds them, the eyes small and deeply sunk in the head, and the tail a mere rudiment. This species is found only in the most gloomy and retired tropical forests of South America. The Indians like its flesh, and are in continual pursuit of it.
In the list of specimens of Mornaidia in the British Museum three species of Bratlyyns and one of Cholarie are given. Several varieties have been described.
B. tridaetylus, the Sloth or Ai. It has a short round heal, furnished with coarse shaggy hair, disposed on the crown in verging rays, like that of the human species ; the face is of a yellowish colour, covered with very short hair, whilst that of the body and extremities is universally long and shaggy ; the eyes are encircled by a brown ring ; the hair of the body varied with irregular patches of dark and light brown, or silvery white ; between the shoulders there is an oval patch of short orange coloured hair, of a finer quality than that found on other parts of the body, and divided in the centre by a longitudinal black stripe ; the throat and breast are frequently of a light straw-colour. The texture
of the hair in altogether peculiar, and more nearly resembles dry hay, or grass shrivelled and withered by the sun, than the hair of ordinary quadrupeds. It is coarse and flattened at the extremity, but as small at the root as the finest spider's web ; and its dry and withered appear ance forms the Ai's principal security against its pursuers, as it renders it extremely difficult to detect it whilst at rest among the branches covered with bark and moss of the same colour. It is only when in motion that it can be readily distinguished from the trunk beneath which it hangs suspended. In other respects different individuals of this species differ considerably from one another in the shades and disposition of their colours, and in the intensity of the mark between the shoulders ; some even want this latter mark altogether, others are of a uniform aah-colour over the whole body, and there are others still which have the hair of the head parted in the centre, and hanging down on each side ; but whether these constitute distinct species, or mere varieties of the common Ai, is a point hitherto undetermined : the cabinets of Europe do not afford sufficient materials for an exten sive comparison, and no naturalist has ever examined the Ais with this view in their native regions.
B. torguatus, the Gipakciou, is a very distinct species, even in the bony structure of its cranium. Its face is naked, and of a black colour ; the hair of its body leas flattened and withered-looking than in the common species ; the forehead, temples, chin, throat, and breast covered with reddish or rust-coloured hair, slightly frizzled ; on the crown of the head it is long and yellow, and on the rest of the body pale orange ; but the most distinguishing mark of the species is a large black collar which completely surrounds the neck. Beneath this outer coat there is an inner one of very fine fur, which is of a dark' brown colour on the collar, but gradually diminishes in intensity towards the croup, where it is entirely white.
Both these species feed upon the leaves of trees, and bring forth but a'single young one at a birth. When in motion in the forests they emit a feeble plaintive cry, resembling the word Ai,' and which is the origin of the name they bear among the Europeans settled in America. They are extremely retentive of life, and have been seen to move their legs and exhibit other' symptoms of vitality a full half hour after being deprived of the heart and other viscera.
B. gala rim, the Yellow-Faced Sloth, is supposed by some to be a variety of B. tridaetylus.