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Cephalaspidiformes (class Cephalaspidomorphi, superorder Cephalaspidomorpha) An order of fish-like, fossil vertebrates which lived during the late Silurian to Devonian Periods. They had somewhat flattened bodies, with a broad head, 10 pairs of ventral gill openings, a ventral jawless mouth, a heterocercal tail, and a single median nostril communicating with a blind-ending nasohypophysial sac. The dorsally located eyes and flat belly suggest a bottom-dwelling way of life. Together with the still-living lampreys (order Petromyzoniformes) they have been placed recently in the class Cephalaspidomorphi, although other classifications have also been made.