50 
M. DESHAYES' TABLES OF SHELLS. 
[Ch. V. 
tions, and having been witness to the great time and labour 
devoted by him to tins arduous work, I feel confidence in the 
results, so far as the data given in his list will carry us. It was 
necessary to compare nearly forty thousand specimens, in order 
to construct these tables, since not only the varieties of every 
species required examination, but the different individuals, also, 
belonging to each which had been found fossil in various loca- 
lities. The correctness of the localities themselves was ascer- 
tained with scrupulous exactness, together Avith the relative 
position of the strata ; and if any doubts existed on these ques- 
tions, the specimens were discarded as of no geological value. A 
large proportion of the shells were procured, by M. Deshayes 
himself, from the Paris basin, many were contributed by dif- 
ferent French geologists, and some were collected by myself 
from different parts of Europe. 
It would have been impossible to give lists of more than 
three thousand fossil shells in a work not devoted exclusively 
to conchology ; but we were desirous of presenting the reader 
with a catalogue of those fossils which M. Deshayes has been 
able to identify with living species, as also of those which are 
common to two distinct tertiary eras. By this means a com- 
parison may be made of the testacea of each geological epoch, 
with the actual state of the organic creation, and, at the same 
time, the relations of different tertiary deposits to each other 
exhibited. The number of shells mentioned by name in the 
tables, in order to convey this information, is seven hundred 
and eighty-two, of which four hundred and twenty-six have 
been found both living and fossil, and three hundred and fifty- 
six fossil only, but in the deposits of more than one era. An 
exception, however, to the strictness of this rule, has been made 
in regard to the fossil shells common to the London and Paris 
basins, fifty-one of which have been enumerated by name, 
though these formations do not belong to different eras. 
It has been more usual for geologists to give tables of cha- 
racteristic shells ; that is to say, of those found in the strata 
of one period and not common to any other. These typical 
species are certainly of the first importance, and some of them 
