A shell
It seems a "true" shell, but it is only a virtual image. You can see another one; or see this one in a cylindrical anamorphosis; or you can go to the website of Atractor to have a look at the way it was built, using a logarithmic spiral.
© Atractor
The image belongs to the sections...:
Animals (Anthology of images)
Lines and curves in 3d-space (3D geometry)
Surfaces in space (3D geometry)
Further information:
http://www.atractor.pt/