In the Brazilian forests they assemble in small parties, and, like the other Marmosets, bound from tree to tree, and keep up a great chattering and whistling, and they cry out with alarm, and soon disperse on the appearance of man within their usual haunts. This fondness for being high up in the woods is carried into their captivity, where they prefer having their little nest up at the top of the cage. In descending from this favourite spot they usually climb down backwards, the tail hanging down. They do not try to stand erect, and, indeed, the position is not natural to them. They like to be caressed and fondled, but they give no such return, and they know those who are kind to them. They dislike strangers usually, and hiss at them. They are very delicate in Europe, as they require a constant high temperature. Cuvier states that these Monkeys have an air sac in the throat, resembling in situation that of the Spider Monkey (_Ateles paniscus_). The Arctopithecini, as a group, have a smooth and rounded skull, large orbits, small brow-ridges, and a large brain-case. The skull is large behind, and the opening for the spinal cord (foramen magnum) is at the junction of the hind third with the two fore thirds of the length of the brain. They have numerous vertebræ in their back-bone, and those in the back and loins number usually nineteen. It is stated by Cuvier that there is an air sac in the neck of the _Midas ursulus_, which communicates with the organ of voice through a space between two of its cartilages. It appears that the hands and feet of the Marmosets have thumbs and toe-thumbs so slightly separable from the fingers and toes that the resemblance to “feet” is decided. This is increased by the fact that the thumbs have claws on them, and the toe-thumb is the only digit with a flat nail, all the rest having claw-like ones. The thumb is really not opposable, but nevertheless the muscles are there to give it movement; the opponens muscle of the thumb is doubtfully present, but the adductors, abductor, and long and short flexors are all there. There is much union of the deeper muscles of the fingers, indicating less independence of movement. In the foot the toe-thumb has no special abductor, and the transversus pedis is absent. CHAPTER XIII. GENERAL REMARKS ON THE QUADRUMANA. The Classification of the Monkeys of the New World--The Geographical Distribution of the Genera--The Fossil Monkeys of the New and Old World and their Alliances--The former old Fauna of Europe, Asia, and Africa--The Resemblance of Quadrumana to other Animals and Man.