The American monkeys differ greatly in size and form. The largest--the savage black howler--is nearly two feet and a half in length of trunk; while the beautiful timid marmoset is so small that it may be inclosed in the two hands. Some have tails twice the length of their bodies; the caudal appendages of others appear to have been docked, or are altogether absent. The long tails of some are prehensile, and have a smooth surface, which enables them to employ it as a fifth hand; others are covered with thick bushy hair, and are employed apparently only in balancing the animal. When night comes they roll themselves into a ball, huddled together as close as may be, to keep themselves warm. Sometimes it happens that a few little monkeys have not been alert enough to get into the ball, and are left shivering outside. They keep up a pitiful howling the whole night through. One family--the Marmosets--have, as has been remarked, claws instead of nails. Others are covered with short, coarse hair; while others, again, have coats of a long, soft silky texture. Some sport among the branches, seeking their food in the daytime; others, again, only come forth from the hollows of trees, where they have their beds during the night season--their eyes being formed, like those of owls, incapable of meeting the glare of day. It is remarkable that the smallest of all--the Hapali pygmaeus, measuring only seven inches in length of body--is among the most widely dispersed, having found its way into Mexico: the only monkey known to have wandered far from the great river-plain. All the monkeys of the New World are arborial; as, indeed, are many of the animals which, in other parts of the world, live entirely on the ground. They are mostly furnished with long, prehensile tails. Some have the under part of the extremity perfectly smooth, so as to serve the purpose of a fifth hand, by which the creatures can swing themselves from bough to bough, and hold on securely while their four hands are actively employed. On passing through an Amazonian forest, sometimes the branches of the trees are seen alive with active little creatures swinging backwards and forwards, climbing up the sipos with the agility of seamen on the rigging of a ship, scampering along the boughs, playing all sorts of antics, or engaged in plucking the juicy fruit or hard nuts to be found in ample abundance, even on the tallest monarchs of the woods. SPIDER-MONKEYS.