In many parts of France—in Auvergne, for example—strata occur of limestone, marl, and sandstone hundreds of feet thick, which contain exclusively fresh-water and land shells, together with the remains of terrestrial quadrupeds. The number of land-shells scattered through some of these fresh-water deposits is exceedingly great; and there are districts in Germany where the rocks scarcely contain any other fossils except snail-shells (_helices_); as, for instance, the limestone on the left bank of the Rhine, between Mayence and Worms, at Oppenheim, Findheim, Budenheim, and other places. In order to account for this phenomenon, the geologist has only to examine the small deltas of torrents which enter the Swiss lakes when the waters are low, such as the newly-formed plain where the Kander enters the Lake of Thun. He there sees sand and mud strewn over with innumerable dead land-shells, which have been brought down from the valleys in the Alps in the preceding spring, during the melting of the snows. Again, if we search the sands on the borders of the Rhine, in the lower part of its course, we find countless land-shells mixed with others of species belonging to lakes, stagnant pools, and marshes. These individuals have been washed away from the alluvial plains of the great river and its tributaries, some from mountainous regions, others from the low country. Although fresh-water formations are often of great thickness, yet they are usually very limited in area when compared to marine deposits, just as lakes and estuaries are of small dimensions in comparison with seas. The absence of many fossil forms usually met with in marine strata, affords a useful negative indication of the fresh-water origin of a formation. For example, there are no sea-urchins, no corals, no chambered shells, such as the nautilus, nor microscopic Foraminifera in lacustrine or fluviatile deposits. In distinguishing the latter from formations accumulated in the sea, we are chiefly guided by the forms of the mollusca. In a fresh-water deposit, the number of individual shells is often as great as in a marine stratum, if not greater; but there is a smaller variety of species and genera. This might be anticipated from the fact that the genera and species of recent fresh-water and land shells are few when contrasted with the marine. Thus, the genera of true mollusca according to Woodward’s system, excluding those altogether extinct and those without shells, amount to 446 in number, of which the terrestrial and fresh-water genera scarcely form more than a fifth.[1] Fig. 18: Cyrena obovata. Fig. 19: Cyrena fluminatis. Fig. 20: Anodonta Cordierii. Fig. 21: Anodonta latimarginata. Fig. 22: Unio littoralis.