This is a "scalariform" Helix aspersa shell from the Smithsonian's Natural History Museum.    From Wordnik.com.  [Interesting animal meme] Reference

A slice across the stem of a tree shows many different tissues with more or less technical names, bark and cambium, medullary rays, pith, and more or less specialised tissue; air-vessels, punctate vessels, woody fibres, liber fibres, scalariform vessels, and other more or less specialised tissues.    From Wordnik.com.  [The Beauties of Nature and the Wonders of the World We Live In] Reference