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Nature is rarely as simple as we might like it to be. The idea that shells are variations on the theme of the logarithmic spiral is so elegant in its simplicity and in its power to explain the diversity of shell shapes that we have trouble modifying it, much less abandoning it. However, the inescapable problem is that as they grow, most shells change shape in ways that do not conform precisely to logarithmic-spiral form. This imprecision might be interpreted as resulting from external conditions. If external conditions did not interfere with orderly shell deposition, for example, mollusks might be able to build their shells "correctly" in accordance with the logarithmic-spiral model. A contrasting way of looking at these deviations is to think of them as normal expressions of additional principles of form that apply to all shells and that show our conception of shells as logarithmic spirals to be inadequate because incomplete.