A sessile (acorn) barnacle in which the rostrum overlaps the rostrolateral wall plates on either side of it, the tips of the terga are not drawn out into a beak, the lines of contact between the terga and scuta are sinuous, and the interior of the base of the shell has numerous centripetal ridges. The scuta have a diagonal line of pits. The inner surface of the scuta and terga is black, and especially in small individuals this shows through to the outside as a dark patch in the center of the scutum where the inner surface is excavated into a deep pit and the scutum is thin. The six wall plates are solid, not filled with hollow longitudinal tubes (except in some young individuals), and do not have fingerlike projecting spines pointing downward. The base is calcified and leaves a calcium deposit on the rock when removed (photo) An extremely common intertidal species, especially in the upper half. Diameter to 2.2 cm, but usually 1.5 cm or smaller. The height is usually about equal to the diameter, unless the barnacles are so crowded that they grow very tall and thin (photo). Walls usually white or grayish white, may have longitudinal ribs (which are often eroded away except in small individuals).