Shell thick, cap-shaped, low conical with the apex closer to the posterior margin or overhanging it. Edge of aperture of irregular shape, matching the surface of the substrate. Sculpture of broad, irregular radiating ribs with narrow interspaces.
exterior of shells white, with reddish brown stains or rays, the interior white with or without brown stains, the edge of the aperture conspicuously tinged with brown.
common size :
up to 20 mm in the Indo-Pacific; the Mediterranean specimen only 3 mm (Barash and Danin, 1992).
The only native species which could create confusion is the pulmonate Trimusculus mamillaris (Linnaeus, 1758), which rarely has the apex overhanging so much and is pure white in color.
BIOLOGY / ECOLOGY
Lives as a commensal attached on shells of larger molluscs, without being host-specific. It is a protandric hermaphrodite (so, large specimens are females) and has long-lived planktotrophic larvae.
shallow subtidal rocky bottoms.