Any South American monkey (platyrrhine) species in either of two families: marmosets (Callitrichidae) or South American monkeys other than marmosets (Cebidae), including capuchins and spider monkeys .
Platyrrhines have a broad nose, with a wide septum separating the outwardly directed nostrils, and relatively unopposable thumbs. Most species have a long tail, which in a few species is prehensile. See also Old World monkey .