Sources : Albirez

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Fish 7.6): Albirez is a marine fish, as Aristotle says, of the genus of fish called rugana. This fish has a very thick skin and is so firm and hard that its skin is used by soldiers for helmets. For its skin by nature has innate strength, so that it is able to resist strong blows. Therefore the soldiers were accustomed to form this skin in the shape of a cap and to put it under their iron helmets, and if any blow from a sword or other weapon should happen to them, it would not cause pain. - [Badke translation/paraphrase]