Manuel,
Your shell may be several things, but I bet it's one of the Lanistes
species.  This is a sinistral (maybe not always) genus of the freshwater
family, Pilidae = Ampullariidae.....

Kurt

At 12:27 AM 7/13/99 +0200, you wrote:
>Dear friends:
>
>        A friend of mine brought me an interesting shell from his trip to
>the Okanvango Delta, in Bostwana. It is sinistral, about 45 mm big, greenish
>in colour, and silky in appearance. It was collected inside the water, so it
>seems to be a fresh water species, rather than a land or tree snail. Do you
>know what species is it? I have no experience nor references for this sort
>of shells.
>        Thank you, and greetings from southern Spain
>
>                                Manuel Jimenez Tenorio
>