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Abstract The growth line of shells showing yearly and daily periodicities of deposition is a useful characteristic for reconstructing prehistoric seasonality of shell gathering. For such investigations, basic studies in recent molluscs are required. This report discusses the results of vital staining and planting (16 days) of the clam, Meretrix lusoria. In the scanning electron photomicrographs, growth lines of variable thickness were observed. They were classified into 5 types (A-E) according to the combination of the thicknesses along the surface of maximum growth (_??_3μ, 3-1μ, <1μ), and those in the "inner zone" of outer layer (_??_ 1μ, <1μ). Among them there were 2 categories: the growth line of the primary order (Types A and B) well discernible in the "inner zone" and more or less conspicuous at the surface of maximum growth, and the secondary one (Types C-E) barely visible or invisible in the "inner zone" and fainter at the surface of maximum growth. In the 2/3 of the specimens, the primary line included only the Type A, the Type B being absent, and in the rest, the Type A formed the great majority in it, a few of the Type B being included. In both cases, the totals of the frequency of these primary lines (only A or A+B) in each specimen were always exactly 16, coinciding the number of days of emplanting. The primary and the secondary growth lines are considered as that of daily and subdaily formation, respectively. The growth increment between the growth lines was seen to be composed of an alternation of lighter and darker zonules.

Daily Growth Lines of the Clam, Meretrix lusoria