XVII . SQUIRREL MONKEYS 559 
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 pre- 
hensile 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 Afe/es, for example. 
The noise produced by these Monkeys is audible for miles, and 
is said not to be due to emulation, 7.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. sciwrea 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. 
Callithriz 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 back- 
wards 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 howlng apparatus like 
