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Orthida (orthids; class Articulata) Extinct order of brachiopods (Brachiopoda), which have biconvex shells, a straight hinge line, and well-developed cardinal areas on both valves (see CARDINAL TOOTH). The pedicle opening notches into both valves and is not usually restricted by plates. The shells are usually impunctate. Orthida range from the Lower Cambrian to the Upper Permian, e.g. Orthis (Ordovician).