The great age of the earth, estimated at 4.5 billion years, can be gauged in the study of the fossil remains of the many creatures which evolved and often perished in the aeons before our own. In this slab of rock from Great Britain are the fossilized remains of several species of invertebrates, such as ammonites, belmnites and brachiopods, dating from the Jurassic Age about 190 to 140 million years ago. The abundance of fossils in this specimen is evidence of the rich diversity of marine life at that time.