Callista chione (L.). is a fairly large, handsome bivalve of the family Veneridae. It is found around the coasts of Devon, Cornwall and Wales, though its distribution extends southwards to the Mediterranean. It is usually uncommon but has been found to occur at the eastern end of Whitsand Bay, near Plymouth at a density of about 1 per m2. Several specimens were taken in 1975 while testing a water lift system for the use of aqualung divers. Most of the shells were observed to bear a noticeable growth check in the form of a ridge several millimetres from the shell margin (Fig. 1). This ridge was not found in the older specimens in the laboratory's museum collection. To try to find out the cause of this growth check a tagging experiment was undertaken in 1976, although it was not possible to measure and mark out large numbers; recaptures in the following two years have been sufficient to provide some evidence for the average growth increment of different sizes of shell.