Broken Mussel Shell

Sometimes something quite common on the beach catches my attention, like this broken piece of mussel shell. I think it's just the way the outer black coating of the shell has worn away, leaving various colours and patterns behind.

Barnacles et al.

 On barnacle covered rocks one can sometimes see tiny periwinkles between and sometimes in the barnacle shells.

In this photo as well as the periwinkles there are also tiny mussels. Notice also the pitting on the barnacle shells. (Click to enlarge photo) The pitting is said to be caused by a lichen.
Note: For more information on the small periwinkles and the pitted barnacles, I highly recommend 'Jessica's Nature Blog.'