48 CLASSIFICATION OF TERTIARY FORMATIONS [Ch. V. 
Rivers scarcely ever fail to carry down into their deltas some 
land shells, together with species which are at once fluviatile 
and lacustrine. The Rhone, for example, receives annually, 
from the Durance, many shells which are drifted down in an 
entire state from the higher Alps of Dauphiny, and these 
species, such as Bulimus montanus, are carried down into the 
delta of the Rhone to a climate far different from that of their 
native habitation. The young hermit crabs may often be seen 
on the shores of the Mediterranean, near the mouth of the 
Rhone, inhabiting these univalves, brought down to them from 
so great a distance*. At the same time that some freshwater 
and land species are carried into the sea, other individuals of 
the same become fossil in inland lakes, and by this means we 
learn what species of freshwater and marine testacea coexisted 
at particular eras ; and from this again we are able to make out 
the connexion between various plants and mammifers imbedded 
in those lacustrine deposits, and the testacea which lived in the 
ocean at the same time. 
There are two other characters of the molluscous animals 
which render them extremely valuable in settling chronological 
questions in Geology. The first of these is a wide geographical 
range, and the second (probably a consequence of the former), 
is the superior duration of species in this class. It is evident 
that if the habitation of a species be very local, it cannot aid us 
greatly in establishing the contemporaneous origin of distant 
groups of strata, in the manner pointed out in the last chapter ; 
and if a wide geographical range be useful in connecting for- 
mations far separated in space, the longevity of species is no 
less serviceable in establishing the relations of strata consider- 
ably distant from each other in point of time. 
We shall revert in the sequel to the curious fact, that in 
tracing back these series of tertiary deposits, many of the exist- 
ing species of testacea accompany us after the disappearance of 
all the recent mammalia, as well as the fossil remains of living 
* M. Marcel de Serres pointed out this fact to me when I visited Montpellier, 
July,182S. 
