In 1877 our Museum received a Cercopithecus died in the Zoological Garden at Rotterdam. Professor Schlegel thought it to be a new species and called it Cercopithecus signatus, but he never described it. As it seems to me to be a very good species I describe it under the name given by Schlegel. It belongs to Schlegel’s section VIII (vide Museum d’Histoire naturelle des Pays-Bas, Simiae, p. 86): Cercopithèques à nez blanc; A: Base du triangle, formé par le champ nasal revêtu de poils blancs, tournée vers le haut. I thus compare it with the two species of this subdivision: Cercopithecus petaurista and Cercopithecus ascanias. I remember that C. petaurista is characterized by having the white color of the sides of the head interrupted by a black band running from the orbits to the sides of the neck, and by another black band running from ear to ear round the vertex (fascia nigra trans caput ab aure ad aurem, see Erxleben p. 36), C. ascanias by a black band running from the nose to the sides of the neck. C. signatus presents no trace of the mentioned black bands and by this character it is very easy to distinguish it from its congeners. Upperparts of body and of tail, underparts of body and inside of legs colored like in C. petaurista. Hind legs colored like forelegs, darker than in C. petaurista and C ascanias: underparts of tail dirty white. Hairs of sides of head grizzled (each hair ringed with white, yellowish and black), abruptly separated from the more reddish colored upperparts of the head by a black band, running from ear to ear over the orbits; it is a band exactly like that in C. petaurista, but does not continue in a black band round the vertex like in C. petaurista. Sides of the muzzle and a few hairs on the anterior part of the lower lip black. The ears seem to be somewhat larger and their inside more hairy than in the two mentioned species.