Tha layer is called callus - It's an extension of the parietal callus. In
several Cymbium species (C. glans, C. cymbium, C. cucumis, C. pachyus) it
can cover the whole shell, overlaying the periostracum. In other sepcies (C.
pepo, C. senegalensis, C. tritonis) it covers about 1/3 to 1/2 of the body
whorl. And in still others (C. olla, C. marmoratum), the parietal callus
doesn't extend onto the body whorl. It is the layer of periostracum beneath
the callus which causes the callus to flake off so easily, especially if you
put them in bleach. In large specimens, particularly of C. glans and C.
cucumis, sand particles and other debris frequently get incorporated  into
the callus, resulting in a lumpy, bumpy shell which in extreme cases can
really be quite bizarre.

Paul M.