The comparison between fossils and medals has frequently been made and fossils have well been styled the "Medals of Creation"; and the significance of fossils as guides to the age of deposits may perhaps be made clearer if we pursue this comparison some way. In the first place there is clear indication of a gradual increase in the complexity of organisation of the fossils as one passes from the earlier to the later rocks, and accordingly the general facies of a fauna is likely to furnish a clue to the age of the rocks in which it is found, even though every species or even genus represented in the fauna was previously unknown to science. So an antiquary versed in the evolution of art or metallurgy, might detect the general age of a medal with whose image and superscription he was not acquainted. He would know that a medal struck in iron was formed subsequently to the bronze age, or that one formed of palladium appertained to the present century. But quite apart from any theoretical knowledge, an antiquary would find as the result of accumulated experience that certain medals are characteristic of certain periods; he would learn that the denarius is characteristic of a different period from that indicated by the coin of the Victorian era, even though he had no knowledge of the technicalities of numismatics. The same is the case with the geologist. He may not be influenced by any knowledge of the evolution of faunas and floras, but actual work amongst the rocks will show him that the trilobite is not found with the belemnite or the ichthyosaur with the elephant, save under exceptional circumstances, which only prove the rule, as for instance when worn bones of ichthyosaurs are washed from their proper strata into gravels with elephant-bones.