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Show 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 ~nee 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. 'fhe 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 50 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 considerably 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 existing 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. 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 freshwater 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 knowledO'e of fossil osteology, conchology, and other branches· but it is desirable that, in the latter science at least, they sho:tld 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 imperfect state of preservation, or the shells happen to belong to ,genera in which it is difficult to decide on the species, except when the inhabitant itself is present, or when any other grounds of a~biguity arise, we must reject, or lay small stress upon,. the. ev1~ence, 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 geoO'raphy ~as reached its present stage of advancement, and endea~our to ~ntrodu~e the same severe comparison of the specific characters, In drawmg all our geological inferences. . Tables of shells by M.Deshayes.-In the Appendix the reader ~Ill find a tabular view of the results obtained by the comparison of mo~·e. than three thousand tertiary shells, with nearly five thous.a nd h. vmg spe ci es, a 11 of w1 n ·c I 1 , w.i t h few exceptions, are con tamed In ~he rich coll~c~ion of~· Desha yes. Having enjoyed :n op~ortun~ty o~ exammmg, agam and again, the specimens n which thts enunent conchologist has founded his identifica- • See vol, i, chnp. vi, Vor,, III, E |