1 literature reference for Callicebus regulus Thomas, 1927:

Author: van Roosmalen, Marc G. M., Tomas van Roosmalen, and Russell A. Mittermeier
Year: 2002
Title: A Taxonomic Review of the Titi Monkeys, Genus Callicebus Thomas, 1903, With the Description of Two New Species, Callicebus bernhardi and Callicebus stephennashi, from Brazilian Amazonia
Source: Neotropical Primates, vol. 10, supplement June, 2002