This cross-section model of a nautilus shell demonstrates one of the nicest natural examples of a logarithmic spiral.
See here for video: https://youtu.be/pzPjslekszg
See here for stable download: https://than.gs/m/932683
Print as oriented to get nice, crisp lines on the inner chambers. Raise the model off the build plate 3mm or so and generate tree supports underneath to support the contoured bottom surface.