This is the typical genus of the American Monkeys. They are distinguished by having a robust body, covered with woolly fur, with a rounded head and a face which, instead of having a protruding muzzle, is more erect and Man-like. They are the commonest Monkeys seen in captivity in our streets. Their tail is long and covered with hair to the tip, and, though prehensile, it is not the perfect substitute for an additional hand noticed in several other genera. Their limbs are only moderately long, and are less slender than in the Spider-Monkeys. The fore-limbs have a well-developed thumb, which, as compared with the length of the hand, is the most {205}Man-like of all the Apes; in some species the nails of the digits are compressed laterally. In the skull the cranial portion exceeds the facial. Professor Mivart observes that in this group the facial part is relatively smaller than in many of the higher Old-World Apes. The skull has no external bony canal (or _meatus_) to the ear; and its frontal bones possess large air-cavities. In the Capuchins the incisor teeth are erect, and are always shorter than the canines. The molars are four-cusped, and have, on their crowns, two transverse ridges and the oblique ridge, already described in the _Lemuroidea_, from the front inner cusp to the hind outer cusp. These animals have also one milk-molar tooth more than in Man. The outer surface of the main brain (_cerebrum_) is almost as much convoluted as in the Old World Apes. The Capuchins range from Costa Rica to Paraguay, and are represented by about eighteen species. They are very gentle and docile animals. F. Cuvier observes in his "Histoire Naturelle des Mammiferes," that of all the Quadrumana--indeed, of all the Mammals--there are none so difficult to characterise as the Capuchins of America, whose colours vary almost with every individual. No two authors agree in the number of species the genus contains. Brisson recognised three, Linnæus four, Gmelin six, Buffon two, and George Cuvier supposed it possible that they all belonged to but one species. Two causes help to produce this diversity of opinion; one is, as remarked above, the natural disposition which these animals have to vary, and to become lighter or darker in colour according to circumstances, and the other is the extremely close relationship that exists between the different species of the genus. Observations, {206}however, are not yet numerous enough, nor exact enough, to enable those who have only studied the species alive in Europe, or had skins, to decide with such imperfect data as to their sex, age, and habitat. Not until some naturalist has made a prolonged study of these animals in their native country, and watched their conduct and relations in the living state, can we hope to attain to any certain knowledge of how many species the genus contains; and of the differences between the old and young of both sexes at different periods from youth to age.