Adult, 8 mm diameter. Grey furrows of juveniles lost from wall plates, and the profile is higher, but not columnar, when crowded. Four wall plates differentiate this species from other British species which have six wall plates. The tergoscutal flaps distinguish it from Balanus crenatus, but they are similar to those of Semibalanus balanoides.
1: carina. 2: tip of tergum. 3: cirri extended, concealing much of tergoscutal flaps.
Mersey Estuary. October 2010.