With the following account of the numerous species of the genera of this family, we come to consider the first section of the Old World, or Catarrhine Monkeys. These are distinguished from their New World cousins, described in the previous pages, by many important and obvious characters. The partition dividing the nostrils is narrow, instead of broad, and the openings of the nostrils themselves are directed downwards and outwards. Certain genera possess also sacs formed by distensible folds of the skin in the cheeks. These "cheek-pouches" serve as a storing-place by the side of the jaws, for food which they cannot masticate at the moment. When this store is disposed of, the folds of skin come together again and give no indication of the presence of the pouch, which, moreover, when full does not interfere with the mastication of other food in the mouth, or with the utterance of the animal's usual cries.
The hind-limbs are never shorter than the fore-; they may be equal in length, but they are generally somewhat longer, the animal being more or less quadrupedal, or very partially erect in gait. Their thumb is not invariably present, but when it is, it is always opposable to its fellow digits. The great-toe is never rudimentary, and is never, as it is in Man, the longest, but is the shortest digit of the foot, and it is capable of free motion to and from the others. All of the digits possess nails. The length of the foot among this group approximates more to the proportions of the foot in Man. The hairs on the arms and fore-arms are directed downwards from the shoulder to the wrist.
The tail in this family varies very much; it may be long or short, or even externally absent, but it is never prehensile. All the species, however, possess "callosities," or hard fleshy pads—often of large size—on the buttocks or seat, which, like the naked skin of the face, are usually brilliantly coloured and often of large size. The perineal region and organs are at certain periods, especially in the females, subject to great turgescence and brilliant coloration.
Besides these external characters, we find, on examining their bony structure, much variation in the skull. Some have a rounded forehead, the ascending portion of the lower jaw being high, broad, and flat, with a large facial angle; in others, we have great production of the upper jaw (the horizontal part of the lower jaw being greater than the ascending portion), and a low facial angle. The cerebral portion of the skull is long and flattened, and the palate long and narrow. The dental formula is I , C , P , M = 32, that of the milk-teeth I , C , M (the forerunners of the permanent pre-molars) = 20, exactly the same as in a Man. The outer lower incisors are equal to, or sometimes smaller than, the inner pair. The permanent canines—which are long and sharp—come in before, or with the posterior molars of both jaws. Between them and the incisors above, and between the canine and the anterior pre-molar below, occurs a gap (or diastema). The anterior upper pre-molar has its outer cusp modified and sharpened; the anterior lower pre-molar has the anterior margin of its crown so shaped as to work "as a scissors'-blade against the posterior edge of the upper canines." (Henley.) The crowns of the molar teeth are long from before backwards, and their fore and hind cusps are united by transverse ridges, a third being present in the same genera, on the posterior lower five-cusped molar.
The nasal bones are often ossified together to form one bone. The surface of the skull is in general oval and smooth, but in some of the Baboons there appear strong ridges over the eyes (hiding the forehead) and along the top of the head, being stronger, when present, in the male than in the female. The external orifice to the ear has a considerable bony tube, or meatus, a distinguishing character which is absent in the New World Monkeys; their tympanic (or ear) cavity being close to the outer wall of the skull. The line of junction (or suture) between the upper jaw-bones, the pre-maxillary and the maxillary, remains unclosed until long after the permanent teeth have come in. Sometimes it remains unclosed throughout life. The foramen for the passage of the spinal-cord, and the condyles for the articulation of the skull with the neck, lie far back.
In the spinal column there are nineteen dorsal and lumbar vertebræ together. The number of caudal vertebræ varies greatly; in some there are as many as thirty-one, in others only three. The posterior ends of the ischiatic bones of the pelvis are rough, flattened, and broad, for the attachment of the fleshy callosities mentioned above.