I couldn't figure it out from the photo, but if it's an owl monkey I wouldn't be surprised. There were already white-faced sakis in there as well as lots of other animal species. Could it be a Geoffrey's marmoset (there are actually 2 in the photo - one on top of the other) as I saw one of those in the walk-through primate exhibit.
does look very lemur-like but I'm inclined to agree with mstickmanp, a pair of grey-legged douroucoulis (so I guess the specific name lemurinus really is appropriate then )
I thinkl it looks like a Doroucouli as well, although I'm never sure about the spelling, and am certainly not clever enough to identify what flavour of Doroucouli it might be. However, it's a funny angle to try to identify it from.
