The grey-crowned titi monkey, Callicebus egeria

Callicebus egeria was described in 1908 by Oldfield Thomas, from specimens collected at Teffé, Middle Amazons, Brazil. He mentions that the species has “a general colouration quite as in C. cupreus, but the crown of the head, instead of being distinctly more fulvous or ferruginous than the back, is less so, being a clear grizzled grey, without even the suffusion of rufous which occurs in both species on the back.
After examining more specimens from the region where the type specimen was collected, he concludes that “the colour of the crown is so variable – rufous or grey – that C. egeria should probably be united with C. cupreus. The cranial differences described in C. egeria do not prove to be as constant as was supposed” (Thomas, 1928a).
Lönnberg (1939) suggests that two specimens from the Rio Purus, Jaburu (Brazil) resemble very much the description of C. egeria, and considers the taxon valid, as a subspecies of Callicebus cupreus. Cruz Lima (1945), Vierira (1955), Cabrera (1958) and Hill (1960) agree all accept egeria as a subspecies of Callicebus cupreus.
Hershkovitz (1988) hesitates to folow these authors, but decides later that egeria is a synonym of Callicebus cupreus cupreus (Hershkovitz, 1990). This is being followed by Groves (2001).

I think that Callicebus egeria might be a valid taxon, but until there is more evidence I would consider it as one of the phenotypical variations of Callicebus cupreus.