it to all the spiral Ammonitoid and Gastropod mollusca11.
For, taking a median transverse section of a Nautilus pompilius, and carefully measuring the successive breadths of the whorls (from the dark line which marks what was originally the outer surface, before it was covered up by fresh deposits on the part of the growing and advancing shell), Moseley found that ãthe distance of any two of its whorls measured upon a radius vector is one-third that of the two next whorls measured upon the same radius vector12. Thus (in Fig. 262), ab is one-third of bc, de of ef, gh of hi, and kl of lm. The curve is therefore a logarithmic spiral.ã
The numerical ratio in the case of the Nautilus happens to be one of unusual simplicity. Let us take, with Moseley, a somewhat more complicated example.
From the apex of a large specimen of Turbo duplicatus13 a {519} line