Greek for "jointed neck", the arthrodires had movable joints between armour plates surrounding the head and body. The mouth is interesting. As the lower jaw moved down, the head shield moved, allowing a large opening. Lacking distinct teeth, like all placoderms, they used the sharpened edges of a bony plate as a biting surface. The eye sockets are protected by a bony ring, a feature shared by birds and some ichthyosaurs. 