The lower jaw has a kind of blunted lobe in front, and the angle of this jaw is broad, triangular, with a rounded lower edge, and it projects backwards beyond the joint which connects the bone with the skull. The cheek bone has those peculiarities which have already been mentioned. The teeth are peculiar, for the first or foremost grinders are smaller than the others, and the second upper grinder is the largest of all. The first grinder on the lower jaw is broader than the rest, and the hinder are the largest, being also cylindrical. [Illustration: COLLARED SLOTH. (From Prince Maximilian of Neuwied’s _Animals of Brazil_.)] It has the general method of living of the Sloths, being perhaps not quite so lively or active as the Unau, and feeds mainly on _Cecropia_ leaves, finishing those of one tree as far as it can before commencing those of another. Like all the Sloths, it has the power of long and sustained muscular action, and can cling on, or grasp, for a very long time without perceptible fatigue, and this gift is associated with a structure of the blood-vessels which supply the muscles, resembling, as we have said, that noticed in some of the Lemurs. The main artery which supplies each of the fore limbs is the axillary, so called from its being found in the armpit or axilla. In quickly-moving animals this vessel reaches into the upper arm, and divides into a few rather large ones lower down, and these give off others, so that a certain quantity of blood is supplied and removed quickly. But in the Sloths the axillary artery divides at once into a number of cylindrical vessels nearly as large as it is, and they are united here and there. These unusual arteries are found in contact with the surface of the muscles, and their branches go in and amongst the muscular bundles. As many as forty-two of these large vessels were counted by Sir A. Carlisle, on the surface of the muscles on the front of the arm, and probably about twenty were inside. These arteries thus carry an immense supply of blood to the muscles, but blood which, although it is finally removed by the veins, does not move very rapidly. In fact, the muscles are turgid with blood. The same arterial structure is seen in the hinder limbs, and the arteries of the thigh form as numerous a set as those of the arm. [Illustration: AI. (From the _Proceedings of the Zoological Society_.)] It seems to be in accordance with careful investigation, to state that the species of Sloth called _Bradypus tridactylus_ (the Three-toed Sloth, or Ai) has too large a meaning, and that it really refers to the Collared Sloth, as well as to others which have been placed in the next genus. It is as well to remark here, that although there are three clawed fingers to the fore limb, there are vestiges of two other ones by their side in the form of two rudimentary metacarpal bones. GENUS ARCTOPITHECUS.--THE AI.[59]