The Polyplacophora are a group of entirely marine molluscs commonly known as chitons or 'coat-of-mail' shells. They are an ancient group, isolated valves having been found in the Upper Cambrian, but the fossil record is generally poor. They are bilaterally symmetrical with an ovoid body that is very greatly flattened dorso-ventrally. The mantle bears 8 transverse shell valves on its dorsal surface.