The Guenons are entirely confined to the African continent, where they range from the Gambia to the Congo, and from Abyssinia to the Zambesi; but the different species are each confined to small restricted areas. Being essentially arboreal, they live entirely in the forest regions, herding together in large troops. They can move from tree to tree with great rapidity, and can climb even on vertical surfaces with surprising quickness. They are abrupt and energetic in their movements, restless, and noisy, incessantly chattering and {43}making grimaces. The latter habit is so characteristic of them that they have obtained from it the name of _Guenon_, by which they are now so generally known, bestowed on them by the French. Their food consists of leaves, birds' eggs, and honey, but pre-eminently of fruits, while they are especially destructive to the ripe grain-fields of the natives near the woods in which they live. They feed voraciously, and carry off all that their cheek-pouches can hold, even after they are satisfied, or if they are called off by the warning cry of the sentinel, who is said to be always placed on guard on some point of 'vantage when the troop is busy with its depredations. The Guenons are not only restless, but very inquisitive; they are, therefore, when young, very easily tamed, and as a consequence they are frequently to be seen as performers in circuses and exhibitions. When aged they are unreliable in temper, and often very ill-dispositioned. They are said, also, to repel with missiles any intruders into the region in which they are established in any numbers. The known species--numbering about forty--have for the purposes of description and easy subsequent discrimination, been arranged into groups (based on a few of their more or less prominent characters) by different zoologists. Of these M. Isidore Geoffroy St. Hilaire, of Paris, and Professor Schlegel, of Leyden, may be specially mentioned; the arrangement of the latter forming a very convenient key for the determination of the species. Among the zoologists who have more recently revised this genus is the well-known Secretary of the Zoological Society of London, Dr. P. L. Sclater, who has to some extent followed and improved upon Professor Schlegel's arrangement of the genus. In the present review, therefore, of the numerous species of this genus, the six groups {44}suggested by Dr. Sclater have been adopted. These are (I.) The Nose-spotted Guenons--_Cercopitheci rhinosticti_; (II.) The Green Guenons--_C. chloronoti_; (III.) The Rufous-backed Guenons--_C. erythronoti_; (IV.) The Black-limbed Guenons--_C. melanochiri_; (V.) The Tufted-eared Guenons--_C. auriculati_; (VI.) The Bearded Guenons--_C. barbati_; and lastly, The Three-cusped Guenons--_C. trituberculati_. GROUP I. CERCOPITHECI RHINOSTICTI. The members of this group have a distinct nose-spot of white, red, or blue. I. THE LESSER WHITE-NOSED GUENON CERCOPITHECUS PETAURISTA.