Found off the coast of Central Vietnam, South China Sea Beach

Photo available:

http://www.badongo.com/pic/867783

These are not exactly shells, but they are shell-related. These are opercula (singular operculum) from Turban Shells (Turbo). The operculum is a kind of "door" attached to the animal's body in such a way that when the animal withdraws into the shell for protection, the operculum is the last thing in, and seals the aperture of the shell for further protection. These particular opercula are from the Goldmouth Turban, Turbo chrysostomus.

Here's a picture of some of these shells without the operculum in place:

http://elrincondelmalacologo.iespana.es/Repetidas/Gasteropodos%20marinos/Turbo%20chrysostomus.JPG

And here's a shell with the operculum in place:

http://study.nmmba.gov.tw/Upload/Biology/金口蠑螺_1.gif

an incredibly timely response with an incredible answer. thank you