The Acre titi monkey, Callicebus acreanus
The species “acreanus” was described in 1952 by Vieira as “Callicebus cupreus acreanus“. The holotype came from Iquiri, Acre, Brasil. The author noted that if differed from the nominal subspecies, Callicebus cupreus cupreus, in “not showing a reddish colouration on the head and back and having the tail much darker on almost the total length”.
Cabrera (1958) regarded the species as a synonym of the earlier described Callicebus toppini (Thomas, 1914) from South-eastern Peru and Bolivia.
Hershkovitz (1963) first synonymized acreanus with Callicebus moloch brunneus, but later with Callicebus cupreus cupreus (Hershkovitz, 1990). Groves (2001) also synonymized it with Callicebus cupreus.
I believe that it is a synonym of Callicebus toppini.