Wikipedia
Gonatocyrtoceras
Gonatocyrtoceras is an oncocerid Nautiloidea genus belonging to the family Acleistoceratidae, that lived during the Middle Devonian. Gonatocyrtoceras is characterized by strongly curved, depressed, breviconic shells. Curvature is exogastric, meaning the lower, or ventral, side is convex in longitudinal profile. The aperture is without a hyponomic sinus. The siphuncle is small and subventral.
Other Devonian Acleistoceratidae are Acleistoceras, Paracleistoceras, and Poteriocerina.