Sources : Perna

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Marine monsters 6.39): Perna is an animal of the sea, and is a monstrous and large animal, as Andelmus says, and is born after the manner of oysters between shells which have a blue color. This monster, between its shells, is clothed with the most noble and brilliant yellow and red, from which costly clothes are made for the beauty and adornment of men and women. For the head-dresses are made of these, which result in the adornment and luxury of women. Thus the skins of the aforesaid animal are held to have a double service, and are to the use of both sexes. - [Badke translation/paraphrase]