Not many shells exhibit the logarithmic spiral in its simplest, planar form. The main living example is the cephalopod Nautilus pompilius. There are many examples (all extinct) in the cephalopod subclass Ammonoidea.
Fossil of the Ammonite Astroceras obtusum.Image from Paleopalace.com,used with permission. This specimen, measuring approx. 4 inchesin diameter, is from the lower Lias formation in Dorset, England.It is about 200 million years old.