This sub-family embraces only one genus, which is very distinct from all the others. The Howlers are the largest of the South American Apes, and are characterised by their thick unwieldy body, their pyramidal head, and small facial angle, owing to their long, somewhat Dog-faced muzzle. The angle of the lower jaw is very large and massive, and their chief characteristic is the conspicuous thickening of the throat, owing to the great enlargement of the hyoid bones--which are widely inflated and cavernous--to form the curious vocal organ which the males of these animals possess, and by which their voice can be so augmented as to be heard at a distance of several miles.[11] The skull is truncated behind {190}in the male (less so in the female) for the reception of the vocal apparatus. Their incisor teeth are small and equal, the canines are prominent and have an oblique ridge across the crown from the outer front, to the inner hind, cusp, and the upper molars are large. The tail is powerful and prehensile, naked towards the tip, where it is tactile and very sensitive. The thumb is movable, the face is naked, and the chin bearded. Some have short, and some have long, fur over their bodies, but it is generally more plentiful about the head. In appearance they are the most unattractive and repulsive of the American Monkeys. Their intelligence is also of a very low order. The roof of the brain-case is depressed; the plane of the opening for the passage of the spinal-cord from the brain is almost perpendicular to that of the base of the skull; the condyles for the articulation of the neck are situated as far back as possible. Sir William Flower, in his valuable monograph on the brain of _Mycetes_, has shown that the frontal lobes are small and the cerebral hemispheres only just cover the cerebellum. In regard to its grooves and convolutions, the main brain (_cerebrum_) of _Mycetes_ can be distinguished from that of all other Monkeys. The whole organ is small as compared with the size of the animal; it wants the roundness and fulness of that of the Spider-Monkeys (_Ateles_) and of the Capuchins (_Cebus_). Its surface markings are comparatively few and simple, and depart remarkably from the ordinary type seen in the order. In the Old World Apes there is a striking similarity in the character of the surface markings of their cerebral hemispheres. There is a slight ascensive development from _Cercopithecus_ towards _Hylobates_; and further complications overlying the same primitive type--such as large proportionate {191}size, and complexity of convolutions--are observed in the Chimpanzee and Gorilla, leading up to the brain of Man. Among the New World genera there is a much greater divergence. Among the Capuchins (_Cebus_), and among them only, there is a precise repetition of the Old World type; but in the genus _Mycetes_ we have modifications in which there is no parallel among the Catarrhine (or Old World) series. There is an