Not one of the numerous kinds of Monkeys which have been noticed in the former chapters has ever been found in the New World--that is to say, on the American continent. The converse is also true, for not one of those which are about to be noticed, and which inhabit the tropical parts of South and Central America, has been seen in any other part of the world. The two groups are not only distinct as regards their geographical distribution, but they are also different in many very important points of their construction and habits. It is evident that, although it may be said that the resemblances between the Baboons, Macaques, and Troglodytes, for instance, indicate some kind of relationship, and suggest a community of origin, there is nothing of the sort to be traced between any Old and New World Monkeys. They seem to have started from different sources. All the Monkeys of the New World have the partition between the nostrils broad, and it separates them widely: they open as it were sideways, and the whole of the lower part of the nose is flat. This peculiarity has given the name to the group, as has been explained in the first chapter, and it is accompanied by some others. Thus, with one exception, the numerous genera of the New World Monkeys have the hinder limbs the longest, and they are wont to go on all-fours, the erect posture being only occasionally adopted by the Spider Monkeys. Their thumbs differ less from the other fingers than do those of the Old World Monkeys, and the toe-thumb is large and movable; no cheek-pouches or callosities are seen in any of them, and only a few have air sacs. It is usual to say that the American Monkeys are known by their prehensile tails, but this is only true in part, for whilst some have this member wonderfully developed and useful, others have it incapable of holding on, whilst a few have barely a tail at all. The teeth are more numerous than in the Apes and Monkeys of the Old World, in one set of New World genera; and they are of the same number in another. In the first instance, there are thirty-six teeth instead of the thirty-two so frequently noticed hitherto, and in the last the thirty-two are differently arranged to those possessed by the Old World kinds. For example, in the prehensile-tailed Howlers, there are thirty-six teeth, or one extra tooth in each jaw and on both sides, over and above the usual thirty-two; and this tooth is a false molar, or one of those between the true grinders and the canine teeth. There are thus three false molars instead of two, as in the Old World kinds, on each side in both jaws.