Answer:
The cockle shell of St. James (Santiago) is based on a legendary story. The adoption of the shell by the pilgrims to the shrine of St. James, is accounted for in a legend, which relates, that when the relics of the saint were being miraculously conveyed from Jerusalem to Spain, in a ship built of marble, the horse of a Portuguese knight, alarmed, we may presume, at so extraordinary a barge, plunged into the sea with its rider. The knight was rescued, and taken on board of the ship, when his clothes were found to be covered with scallop shells.