Pinna nobilis is a type of pen shell, a large saltwater clam, that lives in the Mediterranean Sea. These bivalve creatures produce shells up to a meter (3 feet) long and attach themselves to submerged rocks using strong filaments. These filaments, called byssus or Egyptian flax, are exceptionally fine and can be woven into exquisite silk. This sea silk was light, warm, and strong, with a golden color that was said to never fade. It was very costly among ancient Mediterranean civilizations, including the Egyptians, Greeks, and Romans, and therefore was worn only by the wealthy. Byssus was woven into clothing up until the 20th century, when Pinna nobilis became threatened with extinction from over-harvesting.