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, 1828. 



