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Osteostraci(Cephalaspida; osteostracans; superclass Agnatha) Order of extinct, fish-like, jawless vertebrates, ranging from the Silurian to the Devonian. They had somewhat flattened bodies, a broad head covered in a bony shield, dorsally located eyes with an opening between them for the pineal eye, and a single median nostril in front. At the sides and back of the head shield were areas covered in polygonal plates, possibly covering sense organs. The internal structure of the head resembled that of the extant lampreys. The body was enclosed in bony scales arranged in a series of vertical rows, and there were either one or two dorsal fins and a heterocercal tail. The internal skeleton was partly ossified. Osteostracans were usually small, around 30 cm in length; the dorsally placed eyes and flat belly suggest a bottom-dwelling way of life. See also CEPHALASPIS

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Osteostraci An order of fossil, fish-like, jawless vertebrates that had a flattened head covered by a bony shield with a series of small gill openings on the side. Usually small (30 cm), they ranged from the Silurian to the Devonian Periods. The internal structures of the head resemble those of extant Petromyzonidae (lampreys).