Saimiri saimiri, is a New World Monkey commonly known as the squirrel monkey. They have relatively large brains for their smaller body size, which affects the shape of the skull and head. One of the most noticible feature of its skull is the presence of something called an interorbital fenestra, meaning it has a hole in the bony wall that separates its eyes. In life, the orbits are only separated by a thin membrane covering the fenestra. Its presence is thought to be due to the close approximation of the eyeballs to one another.