When the cross-section of a shell is observed under a microscope, fine striations called growth lines are detectable. A pioneering study of growth lines was made by Davenport (1938), who recorded the daily formation of growth lines by an experiment of notching on the shell margin. However, interest in growth increments in modern and fossil shells came some years later after Wells' (1963) suggestion that the incre ments seen on fossil coral had formed daily and that they appeared to be clustered in yearly periodicity. He was the first to propose the use of growth increments in fossils as "geochronometers." During the following 10 years, the characteristics of growth increments and their periodicity were studied for general use by paleontologists.

Barker (1964) made thin sections of many recent bivalves and pointed out that the finest growth increments were commonly clustered and the clusters made up of about 15, or in some shells 29-30, fine growth increments, suggesting the influence of fort nightly and monthly tidal phenomena. Then Pannella and MacClintock (1968) estab lished a sound base for the study of growth increments, describing the structural char acteristics of growth increments and ordering the growth increments into subdaily, daily and annual periodicities. House and Farrow (1968) demonstrated quantitative analyses of the annual pattern of daily growth increments and the relationships be tween shell growth and environmental parameters.

As reviewed by Clark (1974) and Pannella (1974), the main thrust of these studies during the previous 10 years was the understanding of the implications of these rhyth mic increments: that the increments represented solar days or were formed in a daily periodicity (Barker, 1964; Pannella and MacClintock, 1968; House and Farrow, 1968; Berry and Barker, 1968). However, Evans (1972) reported that the growth pattern of Clinocardillm mualli appears to be wholly correlated with the rhythm of tidal ex posure, and lunar-day periodicity is recognized in some shells of molluscs that inhabit the midtidal zone. Additionally, Neville (1967) indicated two types of growth layers: one created in direct response to environmental fluctuations and the other a direct expression of internal circadian oscillations. The environmental influences exerted on shell growth include not only the tides but also the weather (House and Farrow, 1968; Pannella and MacClintock, 1968). Another aspect of this research on growth increments was the study of the accuracy of periodic formation of growth increments, or the continuity of periodicities, Clark (1968, 1975) noticed missing lines on Pecten diegensis grown in running sea-water aquaria and during some growth checks under experimental conditions, although many pectinids maintained a solar day as their periodicity.

The culmination of these studies of growth increments and geochronometry was a conference (Interdisciplinary Winter Conference on Biological Clocks and Changes in the Earth's Rotation: Geophysical and Astonomical Consequences) held in 1974, and the proceedings published under the title "Growth Rhythms and the History of the Earth's Rotations" (Rosenberg & Runcorn eds., 1975). At this conference, the terminology of growth increments and periodicities were denned, and the micromor phology and rhythmic formation of growth increments in various molluscs and corals together with experimental data were demonstrated.