The Hall of Primitive Mammals traces the lower branches of the evolutionary tree of mammals, including such features as the synapsid opening in the skull (a large hole behind the eye socket for muscles that extend to the jaw, found also in early relatives of mammals), the three middle ear bones (used to classify all mammals), and the placenta. These traits correspond to eating, hearing, and reproduction functions, and each represents an evolutionary branch. Included in this hall are monotremes, marsupials, sloths, and armadillos. Some living animals from these groups, such as the platypus, have so many primitive features that they are called "living fossils."