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Grammoceras
''Grammoceras'' is an extinct ammonite found in Yorks Ravenscar, England, from Jurassic period sediments. Its overall distribution is fairly world wide. ''Grammoceras'' has a thin, discoidally evolute shell with simple, gently sigmoid ribbing and a low ventral keel. ''Dumortiera'' and ''Catulloceras'' from the same age are similar except that ''Dumortiera'' has a more rounded venter and ''Catulloceras'' has a subquadrate whorl section. ''Asthenoceras'' from the slightly younger lower Bajocian may be a subgenus. ==References==
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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「Grammoceras」の詳細全文を読む
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