This is a transverse thin section through the rugose coral Helicelasma sp. from the Sholeshook Limestone near Llanddowror (near St Clears, Carmarthenshire). The Sholeshook Limestone is of Katian age (Upper Ordovician; approximately 450 million years old). Other fossils visible in the image are bryozoans. The specimen is kept under number NMW 2012.28G.1 in the fossil collection of National Museum Wales, Cathays Park, Cardiff.

A thin section is a slice of rock that is so thin (about half the width of a human hair) that it is possible to shine light through it and examine it under a microscope.