The Old World monkeys (Cercopithecoidea) are divided into two subfamilies, the leaf mon keys (Colobinae) and the cheek-pouched mon keys (Cer copith ecinae). The colobines appear first in the fossil record, but the modern forms possess two advanced features, enlarged sali vary glands and a sacculated stomach, which permit them to digest mature leaves. Except for the simakobou (Simias concolor), also known as the pigtailed langur of the Mentawi Islands, all of these have long tails and reduced thumbs. They are largely arboreal but spend some time on the ground, and this may be most pro nounced in the genus Rhinopithecus. The colobines are most diversified in Asia, where the cercopithecines are represented by only a single genus, Macaca. In Africa the colobines are represented by a closely related group cen tering on the genus Colobus, which includes the black and white forms and the red colobus, which is sometimes placed in a separate genus, Piliocolobus. Piliocolobus is often linked to the olive colobus monkeys (Procolobus), also found in Africa. The Asian colobines include Pygathrix, the douc langur; Nasalis, the pro boscis monkey; Rhinopithecus, the snub-nosed langur; Simias, the simakobou; and Presbytis, a diverse group sometimes subdivided to rec ognize Trachypithecus and Semnopithecus as full genera. Although langurs are infrequently used in captive studies, a number of field stud ies of langurs have been reported, with the hanuman langur (Presbytis entellus) perhaps the most frequently studied. It is in this species that males were first described killing infants. Theories of infanticide, as a form of male re productive competition, were developed in the primate literature from this species and are still largely based on it.