The Apes of the Old World differ in many important characters from those of the New. Among the former, as already mentioned, the openings of the nostrils are directed downwards, as in Man; the nose is narrow, and the nostrils themselves are set close together, being separated from each other by a thin septum, or partition, of cartilage. On this account, {127}they have received the name of Catarrhine Monkeys (_Catarrhini_).[7] The New World Monkeys, on the other hand, have the nose flat and the opening of their nostrils directed outwards, and the one nostril widely separated from the other by a broad cartilaginous septum, and they are therefore designated Platyrrhine Monkeys (_Platyrrhini_).[8] The dental formula of the Old World forms is I2/2, C1/1, P2/2, M3/3, making a total of thirty-two teeth in all; but those of the Western Hemisphere differ in having invariably three pre-molars, and sometimes two molars, instead of three, so that they possess either thirty-two or thirty-six teeth altogether. There is always a gap, or _diastema_, in the series of the teeth in front of the upper and behind the lower canines; the latter teeth being taller than the rest. Many of the Catarrhine Apes have large cheek-pouches as well as bare patches, or callosities, often brightly coloured, on the part they apply to the ground when sitting. None of the Platyrrhine group have cheek-pouches or callosities, but in many of them the tail is marvellously prehensile, which is not the case in any of the Old World species. Again, in the Apes of the Eastern Hemisphere, the ear-capsules of the skull have an external bony channel (or _meatus_) for conveying the sound vibrations into the ear, which is absent in the American species. As a rule the Platyrrhine Monkeys have the fore-limbs shorter than the hind-, and are more quadrupedal than those of the Old World. Their thumb is also more like a finger than the same digit in their Eastern brethren. Of the New World Monkeys, the _Hapalidæ_, or Marmosets, have thirty-two teeth, and the _Cebidæ_, with several {128}sub-families, have thirty-six teeth. The former include the Marmosets (_Hapale_) and the Tamarins (_Midas_). The latter comprise the Capuchins (_Cebus_), which may be taken as the representative genus of American Monkeys, the Woolly Monkeys (_Lagothrix_), the Spider-Monkeys (_Ateles_ and the allied _Eriodes_), the Howlers (_Mycetes_), the Sakis (_Pithecia_ and _Brachyurus_), the Night-Monkeys or Douroucolis (_Nyctipithecus_), and the Squirrel Monkeys or Saimiris (_Chrysothrix_), with the allied _Callithrix_.