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SEM observations on orthocerid and ammonoid shell structure are described. The systematic position of the orthocerid cephalopods, all of which in the past had often been referred to the Nautiloidea, is discussed. "Orthocerida" with a capshaped first chamber are referred to the superorder Pseudorthoceratoidea (subclass Nautilomorphi). "Orthocerida" with an ammonoid type of early ontogenesis are named Michelinoceratoidea (subclass Coleoidea). Trematoceratidae n. fam. is described.