PREFACE. 
XV 
studied for six weeks in the museum of M. Deshayes, 
examining his collection of fossil and recent shells, 
and profiting by his instructions in conchology. As 
he had not yet published any of the general results 
deducible from his valuable collection, I requested 
him to furnish me with lists of those species of shells 
which were common to two or more tertiary periods, 
as also the names of those known to occur both in 
some tertiary strata and in a living state. This he 
engaged to do, and we agreed that the information 
should be communicated in a tabular form. After 
several modifications of the plan first proposed for the 
Tables, we finally agreed upon the manner in which 
they should be constructed, and the execution was 
left entirely in the hands of M. Deshayes, in whose 
name they were to appear in my second volume. 
The tables were sent to me in the course of the 
following spring (1831), and additions and corrections 
several months later. They contained not only the 
information which I had expected, but much more, for 
the names of several hundred species were added, as 
being common to two or more formations of the same 
period, whereas it was originally proposed to insert 
those only which were known to be common to two or 
more distinct periods. Thus, for example, more than 
50 shells are now included in the tables, on the 
ground that they are common to the tertiary strata 
both of the London and Paris basins, although they 
