Hairy or Brown Mussel.

Shells quite fragile. Periostracum with it's hairy 'beard' is rapidly lost in the sun on shore, under which is a chalky white shell often with hints of pink.

I have posted a comparative shot of the 4 smaller mussels .
From left to right are the Ribbed mussel, greenlip mussel, Brown hairy mussel and the introduced blue mussel.