Empty Barnacle Shells

Spotted this sculptured-looking shell on the beach at the Temple in Lower Largo. I think that it's a common otter shell (Lutraria lutraria) but it's encrusted with empty barnacle shells, probably from the acorn barnacle (Semibalanus balanoides) which has a diamond shaped aperture.

Close up of the empty barnacle shells which have a cone shaped shell-wall comprising a number of calcareous plates. In acorn barnacles the shell-wall consists of 6 greyish-white plates and there is also a membranous basal plate. The opening at the top of the 'cone' is diamond-shaped.

One of the barnacles has been knocked off but can still see the basal plate by which it was attached (middle left of photo).