Semnopithecinae.—The second sub-family (Semnopitheci nae) includes the genus Semnopithecus, which comprises the sacred monkeys of India, which with their relatives of Asia and the East Indies are commonly termed langurs. In the same sub-family are placed the guerezas of Africa. The most remark able feature of these monkeys is the structure of the stomach which, in correlation with their leaf-eating habits, is greatly enlarged and divided into sacculated compartments. Cheek pouches are absent in the langurs but are said to be present in some guerezas. Semnopithecus has the hind-legs rather longer than the arms and a very long tail. The jaws are relatively short, the thumb shorter than in Cercopithecinae. The hanuman monkey (S. entellus) of central and northern India is one of the best known species. In the large snub-nosed monkeys (Rhinopithecus) of northwestern China and Tibet, the nose is concave with a pro truding upturned point. The proboscis monkey or kehau of Bor neo (Nasalis larvatus) has the most remarkable physiognomy of all monkeys. The nose is enormously elongated, in old males even hanging down over the mouth. Both the snub-nosed and proboscis monkeys have the sacculated stomach and are closely related to the true langurs.
The guerezas are found only in Africa, inhabiting the forests in small troops. They also feed largely on leaves. The most striking difference anatomically is the reduction of the thumb to a vestige, which, however, often bears a minute nail. This re duction parallels in a remarkable manner the condition in the American spider-monkeys. The guerezas are generally known generically as Colobus. In certain species of these monkeys, which are much hunted for their skins, the hair forms a long mantle on the sides and back. The white-tailed colobus (C. caudatus), black in general with white mantle and tail, is a particularly beautiful species. The langurs and guerezas are difficult to keep in captivity and are not often seen in zoological collections.
Origin and Evolution.—The Old World or catarrhine mon keys are widely distributed, mostly in tropical Asia and Africa, a few outlying forms reaching as far north as Tibet, China and Japan, while the baboons extend southward to South Africa. A single species, the Barbary ape of North Africa inhabits the Rock of Gibraltar in Europe. In the Pliocene, however, fossil mon keys of various species have been recorded in England, Ger many, France, Italy, Greece, north Africa and India. In the
Miocene of Tuscany there was a large form (Oreopithecus), in which certain features of the molar teeth suggest remote rela tionship with the anthropoid apes. In the Lower Oligocene of Egypt was found a small lower jaw fragment, Apidium, the lower molars of which may be transitional from the tuberculo-sectorial or primitive mammalian type to the bilophodont form (with two cross-crests) of the Old World monkeys. The group seems to have originated somewhere either in Africa or in the European Asiatic landmass and negative palaeontological evidence indicates their complete absence from America and the Australian region.
No known fossil forms definitely connect the Old World stock with the New World series, tarsioids, lemuroids or tree-shrews. The New World series, for reasons given above, seems entirely independent ; while the known fossil tarsioids appear too peculiarly specialized to be direct ancestors of the Old World stock ; yet such tarsioids as Necrolemur and Microchoerus are the only forms so far known that have even the appearance of evolving toward the catarrhine stage. This is broadly characterized as follows.
Nostrils, closely approximated and opening downward, tending to form a V ; molars with two cross-crests ; dental formula C} PI Mt. Tympanic bone forming a gutter leading to the outer ear ; stomach simple or (in Semnopithecus) highly complex; habits primarily arboreal, the animals climbing as pronograde quadrupeds, mostly on top of the branches. Hands and feet pre hensile. Thumb opposable, more or less flattened nails on all digits. Tail long, short or wanting, never prehensile. Cheek-pouches in most genera. Large callous areas on buttocks, with corresponding flattening on lower ends of pelvis. Placenta double, consisting of a primary and secondary discoidal area with smooth chorion be tween them.
Although we have not as yet been able to trace the direct fossil ancestors of the Old World group into formations older than at most Lower Oligocene, even the known living and extinct tree shrews, lemuroids and tarsioids preserve the broad stages by which arboreal insectivorous mammals with a relatively low type of brain were transformed into monkeys with a relatively high type of brain, with binocular, stereoscopic vision and an ad vanced method of intrauterine nourishment of the young.