The Mathematical Study of Mollusk Shells
-Sir D'Arcy Wentworth Thompson, On Growth and Form
1942 edition, Cambridge University Press, p. 758.
D'Arcy Thompson is referring to a phenomenon which everyone has observed but not everyone has pondered. The shell of a small snail is identical to the shell of a larger one of the same species, except for its size. One is an exact scale model of the other. But a snail does not enlarge its shell by uniform expansion. It adds onto it only at the open end ("terminal growth"). And it does so in such a way that the new shell is an exact scale-up of the old ("unchanging form"). The combination of constraints has a mathematical consequence. Almost all mollusk shells, in all their rich variety of form, must follow the general plan of an equiangular (or, "logarithmic") spiral, or of one of its three-dimensional cousins.
The exceptions include both living and fossil species of Vermicularia and fossil ammonites of the genus Didymoceras.
In this column we will see where the equiangular spirals come into the picture, and how they can be used to generate the underlying geometry of almost every mollusk shell.