Excellent answer, very helpful. In fact, there are some small streams in 
the surroundings. I forward this answer to the owner.

Kind regards

Olivier

Le 02/10/2014 18:43, jan steger a écrit :
> Dear Olivier,
> the sinsitral shell looks to me like a juvenile Physella (or maybe Physa),
> familie Physidae. These snails live in freshwater habitats, but I personally
> also found them in shell grit from coastal environments. It seems to be common
> in some places that land and freshwater shells are transported by streams or
> rivers that enter the sea.
> best regards,
> Jan

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