In the Unau, or Two-fingered Sloth, there is the same general arrangement of the bones and muscles, with some important differences, which result in there being a greater amount of bending and extending of the foot, although the foot rests on its outer edge. A Sloth’s face is short, and there is a broad snub nose, with round nostrils, which are widely open. The cheeks become wide suddenly, and the forehead slopes rapidly backwards, the eyes being wide apart and small, but looking forwards. The head is small and round, and as it is covered with hair behind, it cannot be distinguished well from the upper part of the back of the neck. The expression of the face is always the same, and the method of masticating and eating is disagreeable to observe. The animal having no front teeth, and moving its jaws usually only upwards and downwards, and not from side to side, places the morsel, such as lettuce leaf or carrot, well into its mouth, and chews at it, dragging out the food every now and then, when it is covered with moisture. On examining the skull, the short cut off or truncated appearance of the face is very evident, and it will be observed that the teeth are wanting in the front bones of the face (the pre-maxillaries), and that only the palatal part of these bones exists. The lower jaw is strongly jointed to the upper, and the back part is large: there are teeth at the sides, but there are none in the front part of it. A very singular-looking cheek bone (zygoma) exists on either side. It is not attached behind to the ear bone, so as to cover the jaw muscles, but it has two processes behind--an upper and a lower--which differ in shape and size according to the species. The central bone of the nose does not reach to the nasal outlet, and there is a system of air-cavities which is continued from the nose into the forehead bone. In some kinds, the lower jaw ends abruptly in front, as in the Ai; but in the Unau Sloths it is slightly angular, and projects. The back teeth of the Sloths are very simple, and consist of three structures, called vaso-dentine, hard dentine, and cement, there being no proper enamel. The vaso-dentine is a kind of bony substance in the centre of the tooth, in which there are the passages and tubes of blood-vessels. The dentine is outside this, and consists of more earthy particles than the vaso-dentine, and of fewer tubes; it is all the denser and more resistant. Wearing away more slowly than the vaso-dentine, it forms a ridge which grinds easily. The cement is a kind of bony structure on the outside of the tooth. The teeth of the Sloth continue to grow from below as they are worn above, and there is no entire milk set which are replaced by those of a permanent kind. [Illustration: SKULL OF SLOTH.[57] (From the _Proceedings of the Zoological Society_.)]