The fan mussel, also known as the noble pen shell (Pinna nobilis) is a bivalve shellfish indigenous to the Mediterranean Sea. It is an ordinary animal, with no remarkable traits, with the exception of its unique byssus gland.

The byssus gland produces an extremely fine filament that the fan mussel uses to fasten itself to the sea floor. Because this filament's source is exclusively the byssus gland, it is an especially rare material, and its fineness, lightness and deep golden coloration make it a luxury material in the production of textiles. People have been harvesting this material -- simply referred to as byssus -- for the last 3000 years, and civilizations such as ancient Rome, Chaldea, Persia and China produced and traded in it.

Most byssus trade, at least that taking place in western Europe, died down in the late Middle Ages. There are few remnant examples of byssus cloth from this period, but the few that do exist are mostly women's gloves, woven so fine that they could be packed into an evacuated walnut shell. Byssus trade resumed in Sicily in the last century, but its trade is irregular, and it has no common market price.

The word "byssus" comes from the word used in the Vulgate books Genesis and Exodus to describe fine Egyptian linen. It is understood that the fabric referenced in the Vulgate was not in fact the fabric of the fan mussel; the material now called "byssus" is actually named for the cotton product.

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