From the guereza monkeys (_Colobus_) of Africa, a guenon may be distinguished at a glance by its well-developed thumbs; while these monkeys also differ from guerezas, as well as from their Asiatic relatives the langurs, by having well-developed pouches in their cheeks for storing food. There is, however, another African group of monkeys, the mangabeys (_Cercocebus_), some of which are more difficult to distinguish from guenons, although the majority are recognisable at a glance by their flesh-coloured eyelids, whence the name of white eyelid monkeys. When this character fails, resource must be had to the cheek-teeth, of which the last pair in the lower jaw is less complex in a guenon than in a mangabey; but to examine the back-teeth of a live monkey is not an experiment every one would desire to try! To any one but a specialist, the very large number of representatives of the guenons are exceedingly difficult to class and identify. The matter has, however, been somewhat simplified by arranging the species in thirteen groups, each characterised by the possession of some particular distinctive feature or features. The well-known diana monkey (_C. diana_) represents, for instance, a group in which there are long, upwardly directed, snow-white whiskers associated with blackish or dark red under-parts, a white brow-band, distinctly white areas on the chest and the inside of the arms, a chin-tuft, and a white stripe on the outer side of the thigh. The members of the spot-nosed group, again, as typified by _C. petaurista_, are recognisable at a glance by having a large, heart-shaped white spot on the tip of the nose. The green monkeys, on the other hand, belong to a group (typified by the Abyssinian species) somewhat less easy to recognise, but characterised by the absence of black on the outer surface of the arms, which may be coloured like the body, but are usually, as in the plate, somewhat paler and grey. The face is nearly always black, but may be mottled or freckled with pigment; and, except in one species, the whiskers grow upwards and backwards over the ears. The western species, the aforesaid _C. callitrichus_, lacks the white brow-band present in the Abyssinian species and nearly all the other members of the group; the species forming the subject of the plate being further distinguished by the black-speckled yellowish olive colour of the fur of the back, the long, white whiskers, sharply defined from the hair of the rest of the head, and the presence of a tuft of hair at the root of the tail. [Illustration]