The acorn barnacle, Chthamalus stellatus
Note the kite shaped opening and the horizontal suture meeting
the vertical suture more or less at right angles. This species could be
confused very easily with its close relative C. montagui, and quite easily
with another relative Semibalanus balanoides.
It is very important when doing distribution work on a rock shore to be
able to distinguish the Chthamalus and Semibalanus genera. Help is given
here where the two species are compared.