Mytilus trossulus?

[Emma]

Thanks Dave!  I had a feeling it wasn't going to be as easy as I had hoped.
Well, we went to Copenhagen to visit my sister-in-law who was doing Ph. D. studies in chemistry until Christmas. Before we went I researched the beaches on the internet and found two:

Amager Strand about 5 minutes from the centre on the S-train, and another one in the opposite direction which as 10 mins. away.

We went to Amager Strand, but it was covered in snow. Only the strip by the shore was snow-free.
On seaweed we found tiny, curious ram's horn-like shells and I though they might be Spirula spirula, but I'm not sure.