“A primate with the skull of a Callicebus, the mandible and feet of a marmoset and the tail and teeth of Pseudocebus [=Cebus] ….” (Riberio, 1941 as cited in Hershkovitz1 p. 866). Goeldi's monkeys are the least understood species of platyrrhine. The monkeys' small body size, black coloration, tendency to forage in the low forest understory, and cryptic nature, make them difficult to observe in the wild. Until recently, they had never been the focus of a long-term field study and as a result, little was known of the monkeys' behavior and ecology. Goeldi's monkeys comprise only one recognized species, Callimico goeldii, and have an unusual suite of anatomical and reproductive traits. These traits have created considerable confusion for taxonomists and physical anthropologists.