The Goeldi's Marmoset (or Goeldi's monkey) graphite study is finished (I think.) Dreadful light here today, so may have to re-photograph tomorrow.
After looking at Colchester Zoo's website I was able to identify this species from the reference photos taken some time ago. It is a Goeldi's Marmoset, a South American New World Monkey from the Upper Amazon Basin of Bolivia, Brazil, Colombia, Ecuador and Peru. It is a tree dwelling (arboreal) primate about the same size as other Marmosets and is listed as vulnerable.
Working in graphite again has been very enjoyable and I am going to try to fit one or two in around my regular work.