are enormously enlarged and cavernous, while the jaw—in order to accommodate and protect these various structures—is unusually large and deep. The Howlers are furnished with a fully prehensile tail. The thumb is present. They are described as being the most hideous in aspect of the American Monkeys, and of the lowest intelligence, with which latter characteristic is associated a less convoluted brain than in Ateles, for example. The noise produced by these Monkeys is audible for miles, and is said not to be due to emulation, i.e. not to be comparable to singing or talking, but to serve to intimidate their enemies. The story told of these and other Monkeys with prehensile tails, that they cross rivers by means of a bridge of intertwined Monkeys, is apparently devoid of truth. There are six species, which are Central and South American in range.
The Squirrel Monkeys, genus Chrysothrix, are small creatures with a long head, the occiput projecting. Their tail, though long, has no naked area at the extremity and is non-prehensile. It is a remarkable fact that the proportions of the cranium as compared with the face are greater, not only than in other Monkeys, but than in Man himself. The thumb is short, but not so short as in the Spider Monkeys. The cerebral hemispheres are very smooth; but, as already remarked, this is a matter of size, and not of low position in the series. It may appear at first sight that this statement contradicts the one made concerning the Howlers. But the latter are large Monkeys, and therefore ought, so to speak, to have a more complex brain; but they have not. Like so many of the American Monkeys, the Squirrel Monkeys are gregarious, and, in spite of their tails, arboreal. They are largely insect-feeders, and also catch small birds and devour eggs. There are four species, of which C. sciurea is the commonest, and is constantly an inmate of the Zoological Society's Gardens. Humboldt asserted of it that when vexed its eyes filled with tears; but Darwin did not succeed in seeing this very human expression of an emotion.
Callithrix is a genus not far removed from the last, and, like it, occurs both in Central and in South America. It is chiefly to be distinguished from Chrysothrix by the non-extension backwards of the head, and by the more furry character of the tail. The lower jaw is rather deep, as in the Howlers; but there is not, or there has not been discovered, a howling apparatus like
