Semibalanus cariosus (Pallas, 1788)
Phylum Arthropoda, Subph. Crustacea, Class Thecostraca, Subcl. Cirripedia, Order Thoracica, Family Archaeobalanidae
Acorn barnacle; white to gray; to 60 mm diameter with cascading longitudinal ribs resembling a thatched roof; 6 unequal-size shell plates. Walls permeated with longitudinal channels in irregular rows.
Common, mid- to low intertidal zones; exposed outer coast.
Geo Range: Bering Sea to Morro Bay (central California)
Synonyms: Balanus cariosus
Similar species: Tetraclita is similar in size & also has a thatched appearance but it's usually red and has only 4 shell plates rather than 6.
Image: Mytilus californianus (across top & left)