This genus embraces a large number of species which are characterised by having a thick-set body and short stout limbs, with the thumb set backward. The muzzle is considerably produced and rounded, but the nose does not extend as far out as the plane of the upper lip; the nostrils open in advance of its termination, and are directed obliquely outwards and downwards; their cheek-pouches are large, and their lips thick and protrusile; their eyes are approximated, and look out from below thick and prominent superciliary ridges; their ears are naked and applied flatly to the sides of the head and their hind upper angle is pointed; their callosities, which extend with age, are often surrounded by a portion of the buttocks, which is always nude. The tail is long, short, tufted, or reduced to a mere tubercle, and it may be quite invisible externally. Some have the hair of the head long, and radiating in all directions; others have the face encircled by a kind of mane. In some northern forms, the whole body is covered with a woolly fur, as a protection against cold. {2}In the skull the facial region predominates over the cranial, and the lower margin of the frontal bones are exserted to form a thick prominent ridge over the orbits and nose; the mastoid process on each side of the skull, behind the ear, is very prominent for the attachment of a muscle which assists in opening the mouth and in swallowing their food. Strong muscles also stretch from the back of the head to the spine for the support of the head. The canine teeth are long, and press against the anterior pre-molars of the lower jaw, the position of which is modified or distorted by the pressure, thus enabling these animals to crush and open hard-shelled fruits. Their anterior and median lower molars are four-cusped, while the posterior is markedly larger, and has five cusps and a posterior talon. The carpus, or wrist, possesses the central (_os centrale_) bone, and the fingers have their metacarpal bones elongated. The caudal vertebræ in the species of this genus are usually numerous; even in the short-tailed species they vary from fifteen to seventeen in number, the reduction in the length of the tail being the result of a great diminution in the size, not in the number, of the vertebræ. In the tail of one species (_M. inuus_), however, they are reduced in number to two or three; in the same species the tail lacks the chevron (or V-shaped) bones on its under side, as well as the processes to which the muscles for its movement are attached. Most of the Macaques have a throat-sac, which communicates with the larynx under the thyroid cartilage, and which fills with air, acting as a resonator to their voice.