On the Significance of “Rings” on the Shells of Cardium and other Molluscs

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IN NATURE of February 3, p. 146, I referred to experiments on determining the rate of growth of a fixed population of marked cockles (Cardium edule). In this experiment the box which was fixed in the bed of the River Yealm and contained the cockles was visited monthly, and sometimes at intervals of only a fortnight, for the purpose of measuring the increment in growth since the previous visit. This method of work resulted in an interesting observation on the formation of rings on the shells of the growing cockles. It was found that in the young cockles, i.e. up to about 16 mms. in length, dark rings were formed monthly or fortnightly in a majority of cases, on the shells at the size they were when last measured, but that no similar formation of rings could be detected in the larger and generally older shells. On the other hand, both small and large cockles showed distinct rings after the winter period.