A textile in the Musée des Tissus, Lyon, part of a dalmatic of St. Lambert at Maastricht, had been assigned to the 8th century and said to be of Byssus. The confusion between the ancient use of the term byssus probably for a fine linen and the modern use for the fiber of the Mediterranean mussel pinna marina is discussed. Comparison to these and to Bombyx mori silk, Tussah silk, and spider silk using microscope techniques, identified the fiber as Bombyx silk. The patterned gauze of the textile shows a difference in construction (a different placing of the reed in the loom) that distinguishes it from the complex patterned gauzes of Han China. -- AATA