The Rio Juruá titi monkey, Callicebus torquatus regulus
In 1927 Oldfield Thomas described species originating from Fonte-Boa, Upper Solimoes, Brazil, as a new subspecies, Callicebus torquatus regulus. The taxon has a grizzled brownish back and a contrasted ochraceous crown, just as in C. purinus. However, it is distinguished from the later species in having a blackish breast and belly.
The taxon is now considered to be a species, and its distribution range is thought to be restricted to the Brazilian state of Amazonas, between the upper Rio Solimões, the lower Rio Javarí, and the left (west) bank of the Rio Juruá from mouth at the Rio Solimões to about 7º S.