I chose them originally for their forms. The shell is a complex object in the way that Nature likes complexity. You can turn a shell around, examine it from various perspectives, and each view offers substance that your thoughts can try to grasp.
I wanted something hard to draw, something that would push back as I sought to understand it. I get some of the form of the shell inside my mind successfully only to discover that it has new elements that I didn’t notice before. The more I push, the more it pushes back.
Even without color the shell is mysterious, the ways that light and dark curl around its ridges and projections. Add color and the complexity increases as warm hovers or cool recedes around it in a veil of light.