The outer shell plates have vertical ridges and their pointed tips (along with their conical arrangement) give the barnacle the appearance of a tiny volcano. Coloration is generally grey, often tinged with purple or pink hues. The operculum which seals the top opening is set slightly sunken within the cone formed by the plates. It consists of two pairs of movable plates that open to extend the cirri for feeding. During low tide or in emergencies, the operculum is tightly sealed by a brown or purple flap marked with blue and white patches.