Acorn barnacles (Balanus glandula) are one of the most common barnacle species on the Pacific coast of North America, distributed from the U.S. state of Alaska to Bahía de San Quintín near San Quintín, Baja California. They are commonly found in intertidal waters on mussels, rocks and pier pilings. The acorn barnacle is a moderate-sized barnacle with a diameter of up to 22 mm (0.9 in). The shell is formed by overlapping plates and has a calcareous basis. It has more the shape of a cylinder than the shape of a cone. The white operculum has heavily ridged walls. It can live up to ten years.