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(Linné, 1758)

Description (shell):
Shell is brittle, equivalve; almost equilateral, beaks just behind the midline. Very broadly oval in outline, with a slight twist near the posterior ventral margin. Ligament a thin dark brown band reaching one-third the way to the posterior margin. Lunule is narrow, lanceolate, smooth, about half the length of the ligament pit. Sculpture of evenly spaced, prominent concentric ridges, interspersed with less prominent ridges. Growth stages are not prominent. Right valve with two cardinal teeth, of which the posterior is bifid, and one anterior and one posterior lateral. Left valve with two cardinals, of which the anterior is bifid, and one weak anterior and one weak posterior lateral. The anterior lateral tooth in the right valve is long and thin and has its origin just in front of the anterior cardinal. Adductor muscle scars about equal. Pallial sinus is deep, tongue-shaped, rising diagonally forward, not confluent with the pallial line. Margin is smooth.

Size:
Up to 19 mm in length.

Colour:
White or pale yellow in colour, sometimes with pink or red-brown radiating rays. These rays are clearer near the margins and are of varying width and may coalesce. Periostracum is glossy and transparent. Interior of shell is white or yellow with the external pink or red-brown rays sometimes visible (A. balaustina-photo).

Animal:
Semi-transparent, greyish white with transparent tubes.

Habitat:
Fine sand in deep water.

Distribution:
A rare bivalve (Distr. A. balaustina). It is distributed from the British Isles to the Iberian Peninsula, into the Mediterranean, to the Canary Is., Madeira and along the west coasts of Africa to Sierra Leone and the mouth of the Congo.

Arcopagia balaustina