Honeycomb rock

This is a Palezoic limestone reduced to paper-thin cells by seaside gastropods of some sort.

We craced some of these open at Glyfa, and found some to contain to guilty animals -- apparently they bore through the rock at an early stage of their development, then grow larger INSIDE the rock. They must have some acidic saliva to disolve limestone so efficiently.

I've never seen this in the fossil record, but I assume they must be there?

Each "honeycomb cell" at the top of the photo is about 0.5 cm in diameter.