The inferognathal in the Middle Devonian arthrodire Homostius

The inferognathal or lower jaw bone of the gigantic Middle Devonian arthrodire Homostius (family Homostiidae) is described in detail. The bone is distinctive in shape, being convex posteriorly but concave anteriorly. Inferognathals of the same sinuous shape are rare in arthrodires. They are known also in Angarichrhys and Brachyosteus. An explanation is given of the nature of the supposed shearing surface (described first by Heintz in 1934) at the anterior end of the bone. It is concluded that Homostius had neither teeth nor shearing surfaces. *Middle Devonian, Estonia, Arthrodira, Homostiidae, lower jaw bone, restoration, structure.