The Jurassic fossil sea urchin Clypheus ploti is shown on top of the engraving from 1697 where Robert Plot illustrates the species for the first time in his \Natural History of Oxfordshire\". The species is found in the Cotswolds of Oxfordshire. Robert Plot included them with his 'Stones relating to heavenly bodies, or to Air (moon stone, star stones, Brontiae)'. The five fold echinoid symmetry gives rise to the star shape, and Plot believed that natural forms like this could be generated in the ground (its 'plastic virtue'), just as urchins were in the water. In the past fossils of this species were often also known as \"pound stones\" because their very regular size and mass meant they could be used as a standard measure of weight. In the same book Plot illustrates and describes the first dinosaur fossil (Megalosaurus) as likely the femur of a giant."