The aperture can be closed by an operculum (lid) consisting of a pair of small valves, terga (1), which are often seen edge-on, and a pair of larger kite-shield shaped valves, scuta* (2) 1 Int 01 Barnacle Introduction .
The walls of the shell are made of a carinal plate (3) at the tergal end, a rostral plate (6) at the scutal end, and two or four lateral plates (4&5). The plates interlock with radii (7) overlapping alae (8).