(Linnaeus, 1758)
Description:
Shell ovate, almost pear-shaped, longer than wide, broadest about the middle; rounded, straight or slightly indented anteriorly. Shell endopunctate, whitish, sometimes tinged with yellow or rust, to 33 x 25 mm. Dorsal valve uniformly convex, tapering posteriorly. Ventral valve somewhat deeper, convex, the beak rather short. Foramen incomplete, margined laterally by two small deltidial plates and anteriorly by the umbo of the dorsal valve. Surface covered with fine, radiating striations.
Internal:
No median septum. Brachial loop at first a simple U, becoming annular through the union of the oral processes. Lophophore horseshoe-shaped, with a medial coil. Spicules are present in the body tissue being densely packed in the lophophore and in the tissue covering the mantle canals (T. retusa internal).
Habitat:
It is commonly found attached to the shell of the horse-mussel Modiolus modiolus , as well as on blocks of clinker and boulders, but also to sponges, hydroids and assorted pieces of debris.
Distribution:
Scandinavia to Mediterranean.