All monkeys belong to the Kingdom Animalia, the Phylum Chordata, the Class Mammalia and the Order Primates. In the Order Primates, there are two families comprised of monkeys. These are the Cebidae, or New World monkeys, and the Cercopithecidae, or Old World monkeys. There are several genera of monkeys in each of these families and many species within each genus.
Whether a monkey is classified as an Old World or New World species depends on several characteristics. Among these are the quality of the tail, the structure of the nose and the dental arrangement. New World monkeys tend to have prehensile tails or no tail, while Old World monkeys have tails, but they are never prehensile. Old World monkeys have eight, rather then 12 premolars, and their nostrils face downward, while New World monkeys have nostrils that point upwards.
Common species of Cebidae monkeys are the capuchin monkey and golden lion tamarin. Capuchin monkeys fall into several genera. The genus Sapajus includes the large-headed capuchin and the Margarita Island capuchin, among others. The genus Cebus includes the white-fronted and white-faced capuchins. The golden lion tamarin is a member of the genus Leontopithecus, in which there are four separate, yet similar species of lion tamarins. Common Old World monkey genera include Chlorocebus, which includes the Green monkey species, and Macaca, which includes many species of macaques.