Abstract
It is well known that the dimensions of the ammonite shell conform approximately with the properties of a logarithmic or equiangular spiral. An illuminating account of the logarithmic spiral in its application to the growth of molluscan shells is that given by Sir D'Arcy Thompson (1917, pp. 491–586, and 1942, pp. 748–849). His formula relating the spiral angle of the shell and the ratios of certain linear dimensions must be useful to all interested in shell form in molluscs.

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