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Actinoceriatidae is a family of actinocerids named by Saemann in 1853 for those that grew to have large shells with blunt apecies and large siphuncles with widely expanded segments and a generally arcuate endosiphucular canal system.〔Teichert 1964. Actinoceratoidea, Treatise on Invertebrate Paleontology, part K.(Nautiloidea)〕 Their range is from the upper Middle Ordovician to the Lower Silurian. Actinocerids are generally straight shelled nautiloid cephalopods with a siphuncle composed of expanded segments, typically with thin connecting rings, in which there are internal deposits penetrated by a system of canals Actinoceratids are derived from ''Wutinoceras'', possibly through an early ''Armenoceras'' or through ''Nybyoceras'' 〔Flower 1957.Studies of the Actinoceratida; Memoir 2; New Mexico Bureau of Mines and Mineral Resources, Socorro, NM〕 and give rise to ''Lambeoceras'' and to the Huroniidae. Seven genera are included in the Actinoceratidae, ''Actinoceras'', ''Floweroceras'', ''Kochoceras'', ''Leurorthoceras'', ''Paractinoceras'', ''Saffordoceras'', and ''Troostoceras''.〔 ''Actinoceras'' is the earliest but with a range that carries it into the Lower Silurian. ''Troostoceras'' followed by ''Saffordoceras'' are later Middle Ordovician genera related to early ''Actinoceras''. ''Kochoceras'' followed by ''Floweroceras'' are Upper Ordovian genera related to later ''Actinoceras''. ''Leurothoceras'' and ''Paractinoceras'', both from the Upper Ordovician, are shown to have their source in Middle Ordovician actinoceratids. ''Paractinoceras'' is probably the most distinct actinoceratid from ''Actinoceras''. ''Paractinoceras'' has a long, straight slender shell and a siphuncle that starts off like that in ''Actinoceras'' but becomes narrow like that in ''Ormoceras'' in the anterior part of the phragmocone.〔 Kochoceras has a large blunt shell that expands more rapidly than Actinoceras and is strongly flattened on the ventral side causing possible confusion with Lambeoceras to casual observation. Floweroceras is doubtfully distinct from Kochoceras.〔 ''Leurothoceras'' is synonymous with ''Actinoceras''; ''Saffordoceras'' and ''Troostoceras'' are closely related.〔 ==References== 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「Actinoceratidae」の詳細全文を読む スポンサード リンク
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