(da Costa, 1778)
Description (shell):
Shell thin and brittle, oval, umbones just anterior to midline. Sculpture of numerous, fine concentric lines and grooves. Internal ligament supported in each valve by a triangular, backwardly directed chondrophore; two small cardinal teeth anterior to chondrophore in right valve, a single one in left. Adductor scars and pallial line not readily apparent in fresh material. Pallial sinus broad, lower edge largely fused with pallial line; posteriorly, cruciform muscle scars may be apparent (S. plana-drawing 2).
Size:
Up to 65 mm long.
Colour:
Dull white, yellowish, or grey, usually darkest about the growth lines. Periostracum thin, conspicuous only at margins. Inner surfaces glossy, white or yellowish (S. plana-drawing).
Animal:
(S. plana-animal).
Habitat:
Intertidal only, burrowing in soft substrata in estuaries and tidal flats, in conditions of fluctuating salinity. Often abundant.
Distribution:
Widely distributed, ranges from Norway to the Mediterranean and West Africa (Distr. S. plana).