Burrowed « in estuaries and very sheltered bays, in mud on all coasts […] Clay or mud with abundant detritus. »
(MBSBI). Euryhaline and eurytherm. Intertidal to about 30m deep. Original taxon:
Trigonella plana. Synonyms: callosus, orbiculata, piperata, truncata…
Thick and heavy: beach drift, Cagliari, S. Sardinia. 41mm.
At extreme low tide, on muddy sand, Pointe du Chevet, Saint-Jacut de la Mer, N. Brittany, NW. France. 36-40mm.
« This species is very thin, semitransparent and brittle ; of a somewhat triangular shape ;
greatly depressed, or flat, for the valves are extremely shallow ; of the size of an apricot, or about one and a half inch long, and near two inches broad. The sides are nearly equal.
[…] The beaks are level with the commissure, very small, pointed, and central ; and the margins are plain. […] The hinge […] consists of two minute thin
plate-like parallel teeth, aside of which is a large triangular cavity, and has no teeth. » – E. M. da Costa: Historia naturalis testaceorum Britanniae,
London 1778, via BHL.
