Extinct Palaeozoic class of marine molluscs, with bilateral symmetry, in which the shell shape is variable: most are subtrigonal in cross-section. The walls are generally thick, and composed of laminated calcium carbonate. The apertural end is the widest; the narrowest is the closed apex, which may be pointed or blunt. The juvenile portion is chambered with imperforate septa. Externally the shells are smooth, or ornamented with growth lines and/or longitudinal and transverse ridges. The aperture is usually protected by an operculum possessing paired muscle scars. Individuals are 1–150 mm long. The class ranges in age from Lower Cambrian to Middle Permian, and contains two orders.