Although marine waters are usually found to be supersaturated with respect to both calcite and aragonite, natural precipitation of these carbonate minerals is very restricted. This is apparently due to the occupancy and effective removal of lattice sites by random organic molecules present in natural seawater. Despite this, many marine organisms build external shells or skeletons of calcium carbonate, utilizing various ingenious methods to prevent seawater contamination at the growing shell margin. The periostracum, which apparently evolved as an aid to marginal calcification, has undergone numerous modifications serving important secondary functions in specialized groups.