A prism lights up my 7" African Snail shell

These huge snails offer a substantial feed to whoever is lucky enough to find them.  I discovered  a midden full of discarded shells when riding  a bike around Zanzibar  in aid of a local hospital. The locals thought I was mad to have picked some up of course - but there is no accounting for taste.  The prism is caused by the thick glass on top of my terrace walls.I imagine this huge snail comes under the Wild Wednesday umbrella...