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Cyrtonybyoceras is a slightly exogastric member of the Wutinoceratidae, a family of actinocerids and probably derived from an earlier ''Wutinoceras'' . The shell of ''Cyrtonybyoceras'' is curved slightly upwardly and is slightly compressed. Sutures slope toward the aperture, from the dorsum to the venter. The siphuncle is ventral but not marginal, in general form like that of ''Nybyoceras''. Upper and lower septal necks are recumbent or narrowly free. The canal system is reticular, characteristic of the Wutinoceratidae. Cyrtonybyoceras and Wutinoceras are similar, except for Cyrtonybyoceras being slightly cyrtoconic, and almost intergrade. Cyrtonybyoceras is so far known only from the Whiterock Stage in North America. The genotype, ''Cyrtonybyoceras haesitans'' was named by Teichert in 1933, based on ''Orthoceras haesitans'' Billings 1865, from the Tablehead formation in Newfoundland. Cyrtonybyoceras has also been found in the Antelope Valley Limestone in the Toquima Range in Nevada. ==References== * Flower R.H.,1968;The First Great Expansion of the Actinoceroids, New Mexico Bureau of Mines and Mineral Resources, Memoir 19 Part I, Socorro NM * Teichert, C, 1964; Actinoceratoidea in the Treatise on Invertebrate Paleontology, Vol K; Teichert and Moore, eds; pub GSA & Univ Kansas. 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「Cyrtonybyoceras」の詳細全文を読む スポンサード リンク
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