In the Marmosets, which have only thirty-two teeth, there are only two back grinders in each jaw on each side instead of three, as in the Old World Monkeys, but there are three pre-molars in each jaw on each side. All these distinctions are useful in the classification of these American Monkeys, and therefore they have been divided into two families, one having thirty-six and the other thirty-two teeth, and the first family has again been subdivided into the genera with prehensile tails and those without them. The first to be described are the Cebidæ, and this family contains--first, the genera with thirty-six teeth and with prehensile tails; second, the genera without prehensile tails and the same number of teeth. [Illustration: GROUP OF HOWLERS.] THE HOWLERS.[77] Although articulate speech is denied to the Monkey world, many have very extraordinary voices, the capacity for making a noise being great in them. Thus, the Gorilla has a tremendous voice, and the Gibbons are especially noisy, one of them having been noticed (page 77) to be able to emit something like a series of musical notes. But they are all silent in comparison with the noisiest of all Monkeys--the South American Howlers. The females of this group can make a moderate amount of disturbance, but the males surpass every animal in their prolonged and sustained yelling. Their howlings, commencing often suddenly at the close of day or in the middle of the night, amongst the strange stillness of the great virgin forests, appal the traveller on his first visit. “Nothing,” says Waterton, speaking of the Red Howler, “can sound more dreadful than its nocturnal howlings. While lying in your hammock in those gloomy and immeasurable wilds you hear him howling at intervals from eleven o’clock at night till daybreak. You would suppose that half the wild beasts of the forest were collecting for the work of carnage. Now it is the tremendous roar of the Jaguar as he springs on his prey; now it changes to his deep-toned growlings as he is pressed on all sides by superior force; and now you hear his last dying moan beneath a mortal wound.” Humboldt and Bonpland landed at Cumana, and travelled towards the celebrated cavern of Guacharo, and they saw and heard the Howlers often; and on getting into a cold district their horrible din became worse, and was heard at a distance of two miles. This was near the convent of Caripé, which is more than 4,000 toises (a toise being rather more than our English fathom) above the sea, and where the nights are cold. The animal clearly has earned its appellation of the Howler, and might properly have been called Stentor, as was proposed by a distinguished French zoologist. Stentor was a Greek, whose voice was louder than that of fifty men. But Illiger, probably familiar with the writings of the learned Apuleius--that student of Carthage and Athens who married a rich Roman widow, and was therefore accused of witchcraft, and who wrote the “Golden Ass,” a book