The head of this and all other Howlers has a large black face, and a high receding forehead; the chin recedes much, and there is a great jowl produced by the large bones of the lower jaw. There is a curious swelling at the back of the orbit; and the part of that cavity for the eye which joins the cheek-bone has a round hole in it, as if it had been made by a gimlet. It has two nose or nasal bones, which remain separate, and the forehead (frontal) bone goes so far back that it joins the side (parietal) bones of the skull in a V-shaped suture, or union, and there is not much back to the brain-case, which is depressed in shape, on the whole. They are vegetarians, and yet have very decided canine teeth, those of the upper jaw being large, and they project downwards much lower than the other teeth; and the large lower jaw has evidently quite as much to do with the howling apparatus as with the teeth, for it opens out behind to admit of the great bone of the tongue moving readily within its boundaries. This Howler, like all the others, has good lungs, and a windpipe ending, as usual, in the larynx and its thyroid cartilage (see page 22), as in other Monkeys. The bone at the base of the tongue (the hyoid) is attached to this cartilage, as usual, by a membrane, and instead of being a flat curved bone with two projections on each side, called horns, is swollen out into a bag shape, the horns being very small. The bone in other animals is at the base of the tongue, and this is the case in the present instance, although it is so large, the inside of the hollow being able to contain four cubic inches of water. Now, the air from the upper part of the windpipe can get into this cavity, as there is an opening between it and the upper part of the larynx. Hence the same noise is produced as if the animal howled into a resonant shell. [Illustration: BRAIN OF THE HOWLER.] In order to strengthen the voice, the cartilage of the larynx itself is large and strong, and the so-called ventricles of it are enlarged into air sacs, and they unite in front of the “Adam’s apple.” Besides these there are other sacs connected with the gullet. So that the whole of the front and sides of the neck below and between the sides of the lower jaw are complicated by air sacs and resounding chambers. The breast-bone of the Howler differs in certain respects from that of all other Monkeys, for its upper bone (manubrium) is halved, and each half supports the end of the collar-bone and first rib. Possibly the resulting space may have something to do with the air sacs. [Illustration: YELLOW-TAILED HOWLER AND YOUNG.]