The operculum is the trap door of a shell. It may be of different sizes, but their overall shape is that of a claw. In antiquity the operculum was used as an ingredient in incense. The Babylonian Talmud recorded that Operculum was rubbed with an alkali solution prepared from the bitter vetch to remove impurities. It was then soaked in the fermented berry juice of the Caper shrub or a strong, in order to enhance its fragrance. The operculum was also commonly used as an ingredient in many East Asian incense.