The periostracum comprises an external lamella, 13 nm thick, and one sublamellar layer. Periostracal cells secrete the lamella as preformed periostracal units. The mantle edge gland (meg) produces most of the sublamellar layer. A sequence of formation of periostracal units within the periostracal cells is suggested. Homogeneous inclusions, possibly Golgi derived, fuse into larger, irregular inclusions. Within these inclusions, three-layered membranes, 7 nm thick, arise from the homogeneous material. The membranes fuse in pairs to form the five-layered, 13-nm periostracal units. Acid phosphatase activity has been localised al the surfaces of the periostracal units and might be involved in modifying the units prior to their discharge. Phenoloxidase and polyphenols have been localised in the meg, suggesting that this region is responsible for periostracal sclerotisation. Phenoloxidase activity is present in Golgi, rough endoplasmic reticulum, and apical secretory inclusions in cells in the anterior two-thirds of the meg. Polyphenols are present in apical secretory inclusions, particulary in three or four cells in the posterior meg. This distribution may suggest that phenoloxidase is incorporated into all levels of the sublamellar layer and that sclerotisation occurs subsequently when the enzyme substrate is presented.