Lace doily barnacle

On a section of beach that I hadn't visited before, covered with great sandstone slabs and round, polished rocks, I found many large, empty barnacle shells with lace doily edges, detached from their homes, tossed hither and thither by the tide.

It's not often that I get to examine the underside of a barnacle.

Lacy edge. Compare to the smoother, smaller barnacles on the surrounding rocks.

This is the large thatched acorn barnacle, Semibalanus cariosus. They grow up to a couple of inches across; most of the ones I found were about an inch or so across.

The small barnacles living on the rocks are the common acorns, Balanus glandula, which, at their largest, are less than an inch across; most are much smaller. These, when they die, slowly crumble away, leaving a lacy scar on the rock. The thatched acorn barnacle, instead, has a membranous base, not heavily calcified. When this rots, the shell is released, exposing the frilly rim.

Detail of the shell edge.