molluscan shell, but also, in certain cases, in the little lid or ãoperculumã by which the entrance to the tubular shell is closed after the animal has withdrawn itself within. In the spiral shell of Turbo, for instance, the operculum is a thick calcareous structure, with a beautifully curved outline, which grows by successive increments applied to one portion of its edge, and shews, accordingly, a spiral line of growth upon its surface. The successive increments leave their
Table of Contents
CHAPTER XI THE LOGARITHMIC SPIRAL
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