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Show 50 M. DESHAYESI TAllLES OF SHELLS• [Ch. V. t1· 0ns, an d h avm· g been witness to the great time and la. bour d b h. t this arduous work, I feel confidence m the devote y 1m o . . . 1 resu ts, so ..£1' ar• as the data oo -iven in h1s hst will car.r y us. . It was necessary t o Compare nearly forty thousand speci.m e. ns, m order to construct these tables, since not on~y the v.ari:tl.es of every speci·e s re qui'red examination ' but the different. m.d 1VId.u als, also, belonging to each which had been fo.u~d fossil m vanous loca-lities. 'l'he correctness of the locahtles themselves was ascer~ tained with scrupulous exactness, together with the relative position of the strata; and if any doubts existed o.n these ques~ tions, the specimens were discarded as of no geological value. A large proportion of the shells were procured, b~ M. Desha~es himself, from the Paris basin, many were contributed by d1f~ 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 comparison 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 fiftysix 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 characteristic 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 Ch. V.] DETE~MINATION OF SPECIES. 51 will be seen figured in the plates, illustrative of the different tertiary eras; but we were more anxious, in this work, to place in a clear light, a point of the greatest theoretical interest, which has been often overlooked or controverted, viz., the identity of many living and fossil species, as also the connexion of the zoological remains of deposits formed at successive periods. The value of such extensive comparisons, as those of which the annexed tables of M. Deshayes give the results, depends greatly on the circumstance, that all the identifications have been made by the same naturalist. The amount of variation which ought to determine a species is, in cases where they ap~ proach near to each other, a question of the nicest discrimina~ tion, and requires a degree of judgment and tact that can hardly be possessed by different zoologists in exactly the same degree. The standard, therefore, by which differences are to he measured, can scarcely ever be perfectly invariable, and one great· object to be sought for is, that, at least, it should be uniform. If the distinctions are all made by the same naturalist, and his knowledge and skill be considerable, the results may be relied on with sufficient confidence, as far as regards our geological conclusions. If one conchologist should inform us that out of 119l9l speciesof fossil testacea, discovered in the Paris basin, he has only been enabled to identify thirty-eight with recent species, while another should declare, that out of two hundred and twenty-six Sicilian fossil shells, no less than two hundred and sixteen belonged to living species, we might suspect that one of these observers allowed a greater degree of latitude to the variability of the specific character than the other; but when, in both in~ stances, the conclusions are drawn by the same eminent conchologist, we are immediately satisfied that the relations of these two groups, to the existing state of the animate creation, are as distinct as are indicated by the numerical results. It is not pretended that the tables, to which we refer, comprise all the known tertiary shells. In the museums of Italy there are magnificent collections, to which M. Deshayes had no ·E 2 |