1st, the Sapajous, which have prehensile or grasping tails; 2nd, the
Sagouins, which have ordinary tails, either long or short; and, 3rd, the
Marmosets, very small creatures, with sharp claws, long tails which are
not prehensile, and a smaller number of teeth than all other American
monkeys. Each of these three groups contain several sub-groups, or
_genera_, which often differ remarkably from each other, and from all
the monkeys of the Old World.

We will begin with the howling monkeys, which are the largest found in
America, and are celebrated for the loud voice of the males. Often in
the great forests of the Amazon or Oronooko a tremendous noise is heard
in the night or early morning, as if a great assemblage of wild beasts
were all roaring and screaming together. The noise may be heard for
miles, and it is louder and more piercing than that of any other
animals, yet it is all produced by a single male howler, sitting on the
branches of some lofty tree. They are enabled to make this extraordinary
noise by means of an organ that is possessed by no other animal. The
lower jaw is unusually deep, and this makes room for a hollow bony
vessel about the size of a large walnut, situated under the root of the
tongue, and having an opening into the windpipe by which the animal
can force air into it. This increases the power of its voice, acting
something like the hollow case of a violin, and producing those
marvelous rolling and reverberating sounds which caused the celebrated
traveler Waterton to declare that they were such as might have had their
origin in the infernal regions. The howlers are large and stout bodied
monkeys, with bearded faces, and very strong and powerfully grasping
tails. They inhabit the wildest forests; they are very shy, and are
seldom taken captive, though they are less active than many other
American monkeys.