Comparisons of a moderately worn pm4 in its jaw, and an isolated dpm4 of the Oligocene-Miocene Homunculus (the second tooth originally described as that of a marsupial), indicate cebid identity with affinities nearest Alouatta. Diagnoses of dpm4, pm4 and m1 of all known platyrrhines and comparisons with teeth of strepsirhines and tarsioids reveal that platyrrhines cannot be derived from any known non-platyrrine stock. Speculations on platyrrhine ancestry are reviewed.