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 are 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 
