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Atrina is a cosmopolitan genus of bivalve molluscs characterized by elongated, wedge-shaped shells, distinguished from the genus pinna by the lack of any grooves in the nacreous lining of the shell, and by the central positioning of the adductor scar. As with other pen shells (pinnidae) they commonly stand point-first in the sea bottom in which they live, anchored by net of byssus threads. Atrina is considered to represent the more primitive form within the Pinnidae,; however, both genera are ancient and represented within the fossil record.
A typical species is A. fragilis, found in British waters. A. rigida (Lightfoot, 1786) is found on the southeast coast of North America and in the West Indies. The type species is A. nigra (Dillwyn, 1817, originally P. nigra).
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