The fossils are the most difficult group to identify. They are arranged in biological groups and within each group the genera are to stratigraphically organized as near as is possible. It is not easy to identify fossils below generic level from photograph and naming has been left at this level at nearly every case. This is, nevertheless, a good level to reach. For more detailed classification one of the standard reference works will have to be consulted, or you may need to visit a local museum or university department. I have given an indication of geographical distribution. However, it should be borne in mind that the distribution of species is constantly being modified by fieldwork and research. The expression 'worldwide' is sometimes used and this obviously mean only where favourable strata occur. The geological range of each genus is given with each description.