(Linné, 1758)
Description (shell):
Shell broadly oval to quadrate, umbones distinctly anterior; posterior hinge line straight, posterior margin truncate, anterior hinge line grading into the down-sloping anterior margin. Sculpture of fine concentric striae and bolder radiating lines; prominent posteriorly, where the shell is conspicuously decussate. Growth stages clear. Lunule and escutcheon poorly defined. Each valve with three cardinal teeth: centre one in left valve, and centre and posterior in right are bifid. Pallial line and adductor scars distinct. Pallial sinus U-shaped, not extending beyond midline of shell, but reaching a point below the posterior part of the ligament. Lower limb of sinus distinct from pallial line for the whole of its length (T. decussata).
Size:
Up to 75 mm long.
Colour:
Cream, yellowish, or light brown, often with darker markings. Inner surfaces glossy white, often with yellow or orange tints, and with a bluish tinge along dorsal edge (T. decussata-drawing).
Animal:
Colour cream or grey, its mantle-edges scalloped or edged with white. The siphons are separate throughout their length and fringed with brown at the orifices.
Habitat:
Lower shore and shallow sublittoral, in sand, muddy gravel, or clay.
Distribution:
Patchily distributed in the North Sea (Distr. T. decussata). Distributed south to the Mediterranean and West Africa.