Fossil corals. Assorted specimens of fossilized corals, now extinct, which lived in tropical seas during the Silurian period some 430 million years ago. Conical forms (upper centre, lower centre left) are from Hughley purple shales; tabulate \flattened\" forms (at right and top) are from Wenlock limestone, sites both in England. Corals are colonial marine organisms that live in shallow seas. They secrete an external skeleton of calcium carbonate extracted from the water which, over time, build up into vast coral reefs. The site at which these fossils were found indicate that the British Isles were once much closer to the Equator."