Shell flat and thin, of variable shape, with an apex slightly anteriorly placed; sculpture made up of numerous thin radial costae; external colour greyish with some darker radiating bands; inside with the same colours and pattern than outside, often covered with a blue nacreous layer more or less transparent. The species is named after these dark bluish internal iridescences. The muscular print extends up to the half of the radius; it is uniformly coloured, and bordered by an often brownish margin; the radial pattern appears outward of this margin. The huge synonymy reflects the impressive variability of the species: alba, fragilis, grisea, nacrina, scutellina (little cup), silicina, subplanata…