Verruca stroemia, differs from other British acorn barnacles in having a D-shape operculum made of a single large tergum (1) and smaller scutum (2) which opens along one side.
The four deeply corrugated wall plates are made of a rostrum, carina and the other smaller tergum and larger scutum.

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