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a superorder (according to another system, an order) of bony fishes. The body is naked or has five rows of large bony scales. Thick, rhombic ganoid scales are found only at the base of the upper lobe of the caudal fin. The tail is heterocercal; in unpaired fins the number of rays exceeds the number of actinophores. The mouth is inferior. The skeleton is largely cartilaginous, and ossifications that replace the cartilage are poorly developed. The notochord remains unsegmented for life (the bodies of vertebrae are absent). The superficial bones of the skull are well developed. There is a spiral valve in the intestine and an arterial cone in the heart. The single order, Acipenseriformes, includes the families Chondrosteidae (fossil fishes known from the Lias to the Cretaceous), Acipenseridae, and Polyodontidae.