The shells of marine invertebrates grow incrementally (Wilbur, 1972). When a section of shell is observed under the microscope, the increments are often visible, separated by fine lines (Fig. 1). Studies of these phenomena have focused chiefly on the periodicity of increment and growth-line formation, principally because of its geological or archaeological application as a ‘biological clock’ (e.g. Wells, 1963; Koike, 1973).