The base of the shell attached to the substrate is membranous in some species, as in this image of eight specimens, but others have a white calcareous disk that often remains on the substrate after the barnacle has died and been swept away. The black membranes were ripped, more or less, when these were removed from a limpet shell.

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