XVI PREFACE. 



only occur in the Eocene period to which the strata 

 of those basins belong. The names thus added will 

 increase the value of the tables, and give a more com- 

 plete view of the point to which fossil conchology has 

 now reached ; at the same time, it must be admitted 

 that tables of shells cannot be perfected on this plan, 

 as the science advances from year to year, without 

 soon outgrowing the space which could reasonably be 

 allotted to fossil conchology in a work on geology, for 

 they would soon embrace the names of the greater 

 number of known shells, nearly all of these being 

 common to different groups of strata of the same 

 period. Some of the catalogues which I have given 

 in Appendix II., of fossil shells from the neighbour- 

 hood of the Red Sea, and from some other localities, 

 may illustrate this remark, as they lead us to antici- 

 pate that, at no distant time, we may find a large pro- 

 portion of all the Recent species in a fossil state. 



In treatises on fossil conchology, such as I trust 

 M. Deshayes will soon publish, we cannot have too 

 complete a catalogue of all the species which have 

 been found fossil in every locality, together with their 

 synonyms; but in geological works we can only illus- 

 trate the more important theoretical points by cata- 

 logues of those shells which arc either characteristic 

 of particular periods, as being exclusively confined to 

 them, or which show the connexion of two periods, 

 by being common to each. For this purpose we 



