Laboratory observations on the formation of concentric growth ridges on the shell of small Pecten maximus revealed that their rate of production was related to the size of the shell and to the growth increment in a given period. These factors are related, naturally, and the larger the shell, the greater is the growth increment and the number of ridges produced in a given time. Also, in shells of a similar size the higher the growth rate, the larger is the width of each ridge and the greater is the number of ridges produced. Faster growth is achieved therefore by increasing the width and number of ridges. The rate of ridge production at 10 °C was less than half that at 14·5 °C in conditions which were otherwise identical.