[Balanus amphitrite and similar species]
The Acorn Barnacle is the most common and troublesome family of barnacles. It cements its head directly to the substrate without any stem at all.
This is not an edible family. Most acorn barnacles are around 1/2 inch across or smaller but even the giant barnacles of Antarctica (which may be over 3 inches across) have nothing to eat inside. If you want to eat barnacles, you need Goose Barnacles. Photo © i0124.
More on Barnacles.
Don't bother - nothing to eat here