No idea on the seashell:(

[wolfi]

Hi Alumette,
thanks for your answer. Japs, I love the beaches of Bali with those rather large, ball-shaped grains of sand, got a sample here in a glass........ . You're quite right: Pythia pantherina shows numerous, rather clear defined dark spots, in P. imperforata and P. undata the spots are more vague. I don't know how seldom P. undata is, I only remember a large distribution from Indonesia to Australia.
Kind regards: wolf 
P.s.: just looked for some literature about Pythia in/around Bali and got detailed information and very clear drawings. I think Alumette should compare these infos with her shell.
@diocris: might be, but I'm not quite sure because several species are rather similar.