Cretaceous ammonite, Gault Clay, Folkestone, Kent
Animals such as the extinct Ammonites or their modern relatives, the Nautilus, as well as many other molluscs such as snails and whelks, grow by adding new chambers to their shells. These acted as floats for the Ammonites, and were gas-filled. The pattern is logarithmic, as each chamber is a fixed percentage larger than the previous one.
The image on the right shows the outside of the same animal.