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''Atrina'' is a cosmopolitan genus of bivalve molluscs belonging to the family Pinnidae. 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''). ''Atrina'' is considered to represent the more primitive form within the Pinnidae; however, both genera ''Pinna'' and ''Atrina'' are very ancient. The genus ''Atrina'' is represented within the fossil record from the Triassic period to the Quaternary period (age range: 242.0 to 0.0 million years ago). These fossils have been found all over the world.〔 ==Description== Molluscs within this genus are 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Atrina」の詳細全文を読む スポンサード リンク
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