This family embraces the typical Platyrrhine Monkeys, and to it belongs the great majority of the American species. As already pointed out (p. 127) their nose is flat, and the partition between the nostrils, which open sideways, is very broad, and separates them widely. They are essentially quadrupedal, and walk with the soles of both pairs of limbs flat to the ground. The Spider-Monkeys occasionally, however, assume an erect posture. "They all possess tails, and in some genera (_e.g._, _Ateles_) this organ becomes very flexible and muscular, and the under surface of its curled extremity is devoid of hair and highly sensitive. The tail thus modified is a powerful prehensile organ and serves for a fifth hand." (_Huxley._) In these {151}Monkeys there are no cheek-pouches, nor ischial callosities. Except in the Spider-Monkeys the hind-limbs are longer than the fore-; "while the thumb, even where it is best developed, is capable of but a partial opposition to the other fingers, bending almost in the same plane with the latter, so as to be more like a fifth finger." (_Mivart._) Nevertheless, all its muscles, except the long flexor, are present. The great-toe is large and can be moved from and to the side of the other digits, but is not opposable to them. The skull is smooth and has no muscular crests; the external bony tube to the ear is not ossified. The two extremes in its form are presented by the Howling Monkeys (_Alouatta_) and the Squirrel-Monkeys (_Chrysothrix_), as pointed out by Professor Huxley in his "Anatomy of Vertebrated Animals": "In the former the face is very large and prominent, with a low facial angle. The roof of the brain-case is depressed; the plane of the _occipital foramen_ [for the passage of the spinal cord] is almost perpendicular" to the axis of the base of the skull. "In _Chrysothrix_, on the contrary, the face is relatively small, with a high facial angle; the brain-case is moderately arched;" and the plane of the _occipital foramen_ is horizontal.