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.