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Hematitida is a group of coleoid cephalopods known from the early Carboniferous Period. They are the oldest definite coleoids,〔 although there are controversial claims for even older coleoids from the Devonian.〔 Fossil hematitidans have so far been found only in Arkansas and Utah of the USA.〔 The only family described so far is Hematitidae. ==Characteristics== Some features shared by hematitidans include * a short rostrum, made of aragonite and organic material, and bearing ridges * a short living chamber, only 1.5 to 2 times the length of the last chamber of the phragmocone * a spherical protoconch * a narrow, straight phragmocone * a multi-layered conotheca - the outer wall of the phragmocone * a narrow, ventral siphuncle〔 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「Hematitida」の詳細全文を読む スポンサード リンク
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