The position which the Ateles take in resting is often very curious. The great Apes of the Old World can lie on their backs like a man, and the Monkeys with callosities sit on them, and, drawing up the knees, let the head fall on to them, or on to the breast, bringing the arms forward when they sleep. But the want of callosities, and of the peculiar flatness of back which characterises the Anthropomorpha, prevents the American Monkeys from adopting either of these positions. Many lie on their sides, and others huddle up in parties, but the Ateles often lie across two or three rope-like horizontal stems, with the face looking downwards, a turn being taken by the tail round the support to insure safety. The length of the back has something to do with this, and of course with their extraordinary agility. The dorsal region of the back-bone, or that which bears ribs, is as long in comparison with the other (neck and loin) regions as in any Monkey; indeed, the maximum of length is attained. There are either thirteen or fourteen back-bone pieces (vertebræ), which have ribs attached to them. The lower vertebræ are four or five in number, and the tail is at its maximum of length in relation to that of the body, its pieces (caudal vertebræ) being very complicated near its root. There, eight pieces (vertebræ) are so like those of the back that they have spines, cross processes, of course without ribs, jointing processes, and a similar nervous canal to those which are higher up in the body. The spinal marrow does not go down it, however. Underneath them are the V-shaped or chevron bones. The end bones are short and thick. THE VARIEGATED SPIDER MONKEY.[86]