They are as follow:--The _Sapajous_, whose tails are not only prehensile, but naked underneath, and tubercled near the tips; the _Sajoas_, who possess the prehensile power, but have hairy tails; and the _Sajouins_, whose tails are _not_ prehensile. For want of a better, this classification may be adopted. The Sapajous are subdivided into three genera, of which the Howlers form one. They are so denominated from their habit of assembling in troops, and uttering the most terrible howlings, so loud that the forest is filled with their sonorous voices. Their cries can be heard at a half-league's distance, and produce upon a stranger unaccustomed to such sounds a very disagreeable impression. The unusual strength of voice is accounted for by a peculiar drum-like construction of the _os hyoides_, common to all the genera of Sapajous, but more developed in some than in others; and those in whom the voice is loudest constitute the genus of _Alouatles_, or Howlers. Of the true howlers there are about a dozen species known to naturalists. Most of them are denizens of the tropical forests of Guiana and Brazil; but some species are not so tropical in their habits, since one or two extend the kingdom of the monkeys into Mexico on the north, and southward to Paraguay. Closely allied to the last, are the _Ateles_, or Spider monkeys. These derive their generic name from their singular spider-like appearance-- caused by their disproportionately long and slender limbs, and the great length of their tails. None equal them in the prehensile power of the caudal appendage; and it is of them that that curious story is related-- the story of the Monkeys' Bridge--where it is told how they pass over a stream: a number of the strongest joining their bodies together by means of their long tails, and thus forming a bridge, by which the whole troop are enabled to cross. Of the spider monkeys there are about a dozen species; but three of these have been taken to form one of the three genera into which, as already stated, the Sapajous are divided. These three differ very little from the other spider monkeys, except in being covered with a soft, woolly hair; and, furthermore, in being much more rare than the others; at all events, they are more rarely seen, as they dwell only in the thickest forests, far remote from the habitations of man.