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Pentamerida (pentamerids; class Articulata) Extinct order of brachiopods (Brachiopoda), with thick, strongly biconvex shells. A spondylium developed in the pedicle valve and brachial processes and supporting plates in the brachial (dorsal) valve, forming an internal calcareous box containing the adductor muscles (which close the valves) and diductor muscles (which open the valves). The shell is impunctate. Pentamerida first appeared in the Middle Cambrian and became extinct in the Upper Devonian.