A Unau from Columbia is of a pale and whitey-brown paper colour, darker at the root of the hairs, and it has pale fawn-coloured claws. In all these animals with different kinds of furs, the two-clawed condition is peculiar to the fore limbs only, for on the hinder there are three claws, and it is to be remarked that the hair and skin unite the fingers and toes close up to the base of the claws. The skull of the Unau is rather projecting in front, and not, as it were, quite cut off close; and there is a great gap in the upper and lower gums in front, the incisor teeth being absent, of course. But at the side of the mouth there is a longish tooth above and below, looking like a canine, but really it is the front molar, which in both jaws and on both sides is longer and larger than the others. The under teeth belonging to the lower molar set are placed behind the corresponding upper ones when the mouth is shut. The cheek or malar bone is seen, on looking at the skull, to be separated from the ear bone, and to have a forked end posteriorly, the lower part of the fork extending downwards and backwards. The lower jaw is very straight: it projects a little, in front and behind, where it is jointed with the upper jaw, there is no upright portion or branch, or ascending ramus. The last back tooth is just in front of a curved piece of bone called the coronoid process, the base of which is on a level with the line of the teeth. This Sloth has seven neck bones (cervical vertebræ), and the last one has a very small and rudimentary rib attached to it on either side. There are no less than twenty-three dorsal vertebræ found to be with ribs. The Unau has a clavicle which is much smaller in the other group. It has no tail. The structure of the ankle joint enables it to turn in, even more than that of the Three-clawed Sloth. As the habits of the Unau Sloth are the same as those with three claws, and all live in the same great district, these anatomical distinctions are very interesting, and relate to their remote ancestors, being hereditary legacies, which are of little or no importance in assisting the creature merely to live. One of the differences between the Sloths is singular. The Unaus have a very remarkably formed stomach, which may be said to be double. The first stomach is large and rounded, but it is contracted behind, and then formed into a kind of conical appendix. This appendix is doubled from left to right, and its cavity has a fold at its opening into the stomach. It forms a special part of the first stomach. Then it is to be noticed, that where the food enters the stomach, or at the opening, which is called the cardia, there is a pouch, looking like a bag at the end of the tube which runs down from the gullet to the stomach. This is the second part of the first stomach: and the third is a tube-like space which connects the cardia with the stomach far away to the left. These three cavities form the first stomach.