Verruca stroemia, differs from other British acorn barnacles in having a D-shape operculum made of a single large tergum and smaller scutum which opens along one side.
It lacks an obvious tergoscutal flap, though the reddish interior of the aperture is distinctive.

Full INTRODUCTION TEXT at 01 Barnacle Introduction
Albums with full accounts of all British acorn barnacle species at marinvert.senckenberg.science/image-browse/7215763361…