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.