The observations recorded form preliminary studies which it is hoped will be useful in special investigations on the physiology of shell-depositions in oysters. The distribution of chalky shell-deposits on the internal faces of the valves in young and old O. edulis and O. angulata has been studied, and its average distribution determined in shells of O. edulis of jnedium age from West Mersea and the Fal Estuary.

The distribution of this material in the shells of young O. edulis and in O. angulata is such that there can be little doubt that the function of the chalky deposit is to fill in rapidly grooves, hollows, and other spaces, which are inimical to efficient functioning in the changing needs of the individual.

In O. edulis of medium age large deposits occur regularly in the shell adjacent to the exhalent chamber and smaller ones on the border of the shell in the region of the inhalent chamber. The hypothesis is advanced that these regular deposits can be explained in the same way as those more easily understood by assuming that in a high percentage of shells of medium age, the contour of the shell in the region of the exhalent chamber and other parts is such that rapid extension and contraction of the mantle cannot be effected; deposition of chalky shell-material modifies this contour of the shell with certainty in some cases, but hypothetically in others, so that the mantle can be extended and retracted with a maximum of efficiency.