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Elephant and mammoth tusk ivory comes from the two modified upper incisors of extant and extinct members of the order Proboscidea. Mammoths have been extinct on mainlands for 10,000 years, and on islands for 4,000 years. Because of the geographical range in Alaska and Siberia, ''Mammuthus primigenius'' tusks have been well preserved. Therefore, ''Mammuthus primigenius'' is the only extinct Proboscidea which consistently provides high quality, carvable ivory. An African elephant tusk can grow to 3.5 metres in length. Enamel is only present in the tusk tip in young animals. It is soon worn off and not replaced. Whole cross-sections of proboscidean tusks are rounded or oval. Dentine composes 95% of the tusk and will sometimes display broad concentric bands. Cementum, which can be thick in extinct genera, covers the outside of the tusk. Cementum can present a layered appearance, particularly in mammoth. Polished cross-sections of elephant and mammoth ivory dentine display uniquely characteristic Schreger lines. Schreger lines are commonly referred to as cross-hatchings, engine turnings, or stacked chevrons. Schreger lines can be divided into two categories. The easily seen lines which are closest to the cementum are the outer Schreger lines. The faintly discernible lines found around the tusk nerve or pulp cavities are the inner Schreger lines. The intersections of Schreger lines form angles. These Schreger angles appear in two forms: concave angles and convex angles. Concave angles have slightly concave sides and open to the medial (inner) area of the tusk. Convex angles have somewhat convex sides and open to the lateral (outer) area of the tusk. Outer Schreger angles, both concave and convex, are acute in extinct proboscidea and obtuse in extant proboscidea.〔(【引用サイトリンク】title=Ivory Identification Guide - U.S. Fish and Wildlife Service Forensics Laboratory )〕 China was the biggest market for poached ivory but announced they would phase out the legal domestic manufacture and sale of ivory products in May, 2015, and in September 2015 China and the U.S.A. "said they would enact a nearly complete ban on the import and export of ivory." (). ==See also== *Ivory trade 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「Elephant and mammoth ivory」の詳細全文を読む スポンサード リンク
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