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Tectus niloticus
''Tectus niloticus'', common name the commercial top shell, is a species of sea snail, a marine gastropod mollusk in the family Tegulidae.
This species is a very large (up to 15 cm) Indo-Pacific top shell, which has a very thick inner layer of nacre. This species is commercially exploited to make mother of pearl buttons, mother of pearl beads, pendants and so on. In 2006, for example, the sole commercial export of the Wallis and Futuna Islands was 19 tons of "Trochus" shells, valued at US$122,000.〔 〕
==Description==
The length of the shell varies between 50 mm and 165 mm, its diameter between 100 mm and 120 mm. The large, ponderous, shell has a conical shape, appearing subperforate. It is covered by a corneous striate, brown or yellowish cuticle usually lost on the upper whorls. Its color beneath the cuticle is white, longitudinally striped with crimson, violet or reddish brown. The base of the shell is maculate or radiately strigate with a lighter shade of the same. The spire is strictly conical. The apex is acute, usually eroded. The shell contains 8-10 whorls. The upper ones are tuberculate at the sutures, and spirally beaded, the following flat on their outer surfaces, smooth, separated by linear suture. The body whorl is expanded, dilated and compressed at the obtuse periphery, more or less convex below, indented at the axis. The umbilical tract is covered by a spiral pearly deeply entering callus. The aperture is transverse and very oblique. The columella is oblique, terminating in a denticle below, and with a strong spiral fold above, deeply inserted into the axis. The operculum is circular, thin, corneous, orange-brown, and composed of about 10 whorls.〔(Tryon (1889), Manual of Conchology XI, Academy of Natural Sciences, Philadelphia )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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