Now this is really special! The chambered nautilus is a cephalopod mollusc, related to the octopus and the squid, but it grows a shell like a snail shell in the form of a perfect Archimedean spiral (see also the Golden Ratio in the Geeky section). As it grows, the nautilus closes off chambers that it uses for flotation, each proportionately larger than the last.
30 years ago I bought one of the shells in Solomon Islands and a radiographer friend X-rayed it for me. Now I've brought together a photo of the shell and the X-ray for your enjoyment of their geometrical perfection.