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''Pisidium clessini'' is a species of extinct freshwater bivalve from family Sphaeriidae. ==Description== The shell has an oval form with somewhat prominent umbo, that is situated behind the middle. There is a distinct concentric rib structure. The ribs are sharp and are separated by relatively big spaces. There are two (sometimes three) strong accentuated ribs near the Umbo, that gradually become less accentuated to the sides. These ribs are a distinct characteristic of the species. The ribs can be faded in certain cases, but the accentuated ribs are always there. Radial ribs are not present. The inner rim of the shell is smooth and is not crenulated. The shell of this species has a somewhat variabel shape and sometimes does look a lot like ''Pisidium amnicum''. Measurements of the shell: *length: up to 7,8 mm. *width: up to 6,5 mm. *semi diameter: up to 2,2 mm. 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「Pisidium clessini」の詳細全文を読む スポンサード リンク
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