Class of primitive, almost bilaterally symmetrical, univalved molluscs, whose limpet-shaped shell of calcium carbonate consists of an external periostracum (the outer, non-calcareous layer), a prismatic layer, and an internal, nacreous deposit. The internal organs are metamerically segmented, and the circular foot and radula are ventrally placed. Typically, Monoplacophora have paired muscle scars. All are marine, benthic filter-feeders. Fossil forms inhabited shallow water, but modern forms occur in deep water. They first appeared in the Lower Cambrian.