Are you certain they are not just part of the concrete defence wall?
You know the marine lake, there's a walkway right around it. On the outer side towards the sea, the council have dropped big stones. These shells/fossils are embedded in some of those.
I mainly saw them on the side facing the Dee.
I'm not talking about the promenade where there's benches and people park their cars, it's the little pathway around the lake.