A Histological investigation has been made of growth and repair of periostracum in Mytilus edulis. Physical rather than chemical differences in the processes of secretion can account for the variations in the structure of periostracum which appear during normal growth and in that which follows damage. Amœbocytes take part in the production of periostracum by supplying material for the secretory cells. Reasons for the presence of other granular cells near the site of secretion are also suggested. The periostracum is secreted solely by the epithelium of the inner surface of the outer mantle fold, although initially it adheres to the epithelium of a process of the middle fold. Consideration of the nature of the connection of the periostracum with its retractor muscles leads to the hypothesis that the cells of both epithelia move with the periostracum towards the mantle edge. The tissue where the two epithelia meet appears to be meristematic.