any marine snail of the family Turbinidae (subclass Prosobranchia of the class Gastropoda) that has a wide aperture in the first whorl of the stout shell, which is topped by a bulbous, turbanlike coil. The shell may be beaded, knobbed, or ridged. The largest species of turban shell is the 20-centimetre (8-inch) green turban (Turbo marmoratus), native to the East Indies and Australia; its broad, round “cat's-eye” operculum (lid for closing the aperture) is used for making buttons.