The acorn barnacle, Semibalanus balanoides
Note the diamond shaped opening and the horizontal suture meeting the
vertical suture at an oblique angle. This species could be confused very
easily with its close relatives Chthamalus stellatus and C. montagui.
It is very important when doing distribution work on a rocky shore to
be able to distinguish the Chthamalus and Semibalanus genera.
Help is given here where the two species are compared.