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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Ascocerida ： ウィキペディア英語版 Ascocerida The Ascocerida are comparatively small, bizarre nautiloids known only from Ordovician and Silurian sediments in Europe and North America, uniquely characterized by a deciduous conch consisting of a longiconic juvenile portion and an inflated breviconic adult portion that separate sometime in maturity.〔W.M . Furnish and Brian F Glenister; Nautiloidea-Ascocerida, K261-K277; Treatise on Invertebrate Paleontology , part K, Nautiloidea; Geological Society of America and the University of Kansas Press, 1964〕 ==Morphology== The juvenile portion of an ascocerid consists typically of a narrow, circular cyrtocone that underwent periodic truncation. Cumulative length, included broken off segments, may have reached about in the largest. The siphuncle is located half way between the shell axis and the venter, is thin walled and tubular with short, orthochoanitic septal necks and segments that are only slightly inflated. The juvenile portion is known as the deciduous conch,〔 as it is shed in a similar manner to deciduous trees losing their leaves. The adult, or mature ascoceroid conch, as it is referred to, consists typically of an expanded exogastric brevicone with special unique features. The apical end is formed by the septum of truncation which is about three times s thick as the normal internal septa and about as thick as the external shell itself. The septa become confined to the dorsal side of the shell resulting in a series of dorsal chambers, or camerae, that provided stabilizing buoyancy.〔 The ascocerid shell, or conch, is the thinnest and most fragile of any orthocone or cyrtocone of comparable size.,〔〔Rousseau H Flower; p 542, Ordovician Cephalopod Faunas and Their Role in Correlation; in The Ordovician System: proceedings of a Palaeontological Association symposium, University of Wales Press and the National Museum of Wales, Cardiff, 1976〕 which accounts for their rarity. Often it is only the mature, ascoceroid portion that is found. 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「Ascocerida」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.