To make a seashell shape, sweep a circle around a helix while applying an exponential decrease in scale, such that successive turns are tangent to one another.
Seashell Tangent 01 has a decrease angle of pi/8, and a rise angle also of pi/8, so that the bottom of the swept circle stays tangent to the XY plane. It's colored by Gaussian curvature, and available in Full Color Sandstone.
The toolkit we use to turn these equations into printable objects is called
Anethole, and is available on GitHub.