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''Armenoceras'' is the principal genus of the Armenoceratidae, ranging from the late Whiterockian Stage in the early Middle Ordovician, through the remainder of the period and on into the Upper Silurian. The shells of ''Armenoceras'' are straight and medium to large in size with a circular to subcircular cross section. The siphuncle is large, located subcentrally to resting on the ventral margin. Segments are wider than long, broadly expanded into the camerae. Septal necks short with wide brings that may be in contact with posterior surface of septa. In most the canal system within the siphuncle in is of the double arc type. Cameral deposits are rare. ''Armenoceras'' is derived from ''Wutinoceras'' through a thinning of the connecting rings and a simplification of the endosiphuncular canal system. The earliest ''Armenoceras'' known comes from the lower Whiterock equivalent in northern China and Korea but is unknown in North America until the end of the Middle Ordovician when it appears in Red River faunas. Worldwide, in addition to east Asia and North America including Greenland, ''Armenoceras'' has been found in northern Europe, Russia, and Australia. ''Armenoceras'' is thought to have given rise to ''Nybyoceras'' in the Chazy (Flower 1968), although the reverse was earlier suggested in Teichert (1964), and to ''Selkirkoceras'' in the Eden-Red River Stage in the Upper Ordovician. Possible Silurian derivatives include ''Megadiscosorus'', ''Monocyrtoceras'', and ''Elrodoceras'' although some may turn our to have ''Nybyoceras'' as the progenitor. ==References== *Flower, R.H, 1957, Studies of the Actinoceratida, New Mexico Bureau of Mines and Mineral Resources, Memoir 2. *Flower, R.H. 1968, The First Great Expansion of the Actinoceroids; New Mexico Bureau of Mines and Mineral Resources, Memoir 2, PtI. *Flower, R.H, 1978, Ordovician Cephalopod Faunas and Their Role in Correlation; in The *Ordovician System: proceedings of a Palaeontological Association symposium; Bassett, M.G. (Ed) *Teichert, C, 1964, Actinoceratoidea, in the Treatise on Invertebrate Paleontology, pub Univ of Kansas and the GSA, Vol K, p K206-207; Fig 137 p K200-201. 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「Armenoceras」の詳細全文を読む スポンサード リンク
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