Ch. V.] 
IN CHRONOLOGICAL ORDER. 
49 
species of several other classes. We even find the skeletons of 
extinct quadrupeds in deposits wherein all the land and fresh- 
water shells are of recent species*. 
Necessity of accurately determining species. — The reader 
will already perceive that the systematic arrangement of strata, 
so far as it rests on organic remains, must depend essentially 
on the accurate determination of species, and the geologist 
must therefore have recourse to the ablest naturalists, who have 
devoted their lives to the study of certain departments of 
organic nature. It is scarcely possible that they who are con- 
tinually employed in laborious investigations in the field, and 
in ascertaining the relative position and characters of mineral 
masses, should have leisure to acquire a profound knowledge of 
fossil osteology, conchology, and other branches ; but it is 
desirable that, in the latter science at least, they should become 
acquainted with the principles on which the specific characters 
are determined, and on which the habits of species are inferred 
from their peculiar forms. When the specimens are in an im- 
perfect state of preservation, or the shells happen to belong to 
genera in which it is difficult to decide on the species, ex- 
cept when the inhabitant itself is present, or when any other 
grounds of ambiguity arise, we must reject, or lay small stress 
upon, the evidence, lest we vitiate our general results by 
false identifications and analogies. We cannot do better than 
consider the steps by which the science of botanical geography 
has reached its present stage of advancement, and endeavour to 
introduce the same severe comparison of the specific characters, 
in drawing all our geological inferences. 
Tables of shells by M. Deshayes. — In the Appendix the reader 
will find a tabular view of the results obtained by the compa- 
rison of more than three thousand tertiary shells, with nearly five 
thousand living species, all of which, with few exceptions, are 
contained in the rich collection of M. Deshayes. Having enjoyed 
an opportunity of examining, again and again, the specimens 
on which this eminent conchologist has founded his identifica- 
* See vol. i. chap. vi. 
Vou III. E 
