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''Pinna'' is a genus of bivalve molluscs belonging to the family Pinnidae. The type species of the genus is ''Pinna rudis''. The most completely studied species in the genus is ''P. nobilis'', a Mediterranean pen shell which was historically important as the principal source of sea silk. ==Description== These pen shells can reach a length of about . They are characterized by thin, elongated, wedge-shaped, and almost triangular shells with long, toothless edges. The surface of the shells shows radial ribs over their entire length. ''Pinna'' is distinguished from its sibling genus ''Atrina'' by the presence of a sulcus dividing the nacreous region of the valves, and the positioning of the adductor scar on the dorsal side of shells. These bivalves most commonly stand point-first in the sea bottom in which they live, anchored by a 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「Pinna (bivalve)」の詳細全文を読む スポンサード リンク
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