must be referred to time, or the different epoch at which the upper marine beds of the Paris basin and the Faluns of the Loire originated. _Mayence._--The true chronological relation of many tertiary strata on the banks of the Rhine has always presented a problem of considerable difficulty. They occupy a tract from 5 to 12 miles in breadth, extending along the left bank of the Rhine from Mayence to the neighbourhood of Manheim, and are again found to the east, north, and south-west of Frankfort. In some places they have the appearance of a freshwater formation; but in others, as at Alzey, the shells are for the most part marine. _Cerithia_ are in great profusion, which indicates that the sea where the deposit was formed was fed by rivers; and the great quantity of fossil land shells, chiefly of the genus _Helix_, confirm the same opinion. The variety in the species of shells is small, while the individuals are exceedingly numerous; a fact which accords perfectly with the idea that the formation may have originated in a gulf or sea which, like the Baltic, was brackish in some parts, and almost fresh in others. A species of _Paludina_ (fig. 154.), very nearly resembling the recent _Littorina ulva_, is found throughout this basin. These shells are like grains of rice in size, and are often in such quantity as to form entire beds of marl and limestone. They are as thick as grains of sand, in stratified masses from 15 to 30 feet in thickness. [Illustration: Fig. 154. _Paludina._ Mayence.] That these Rhenish tertiary formations agree more nearly with the Upper Eocene deposits above enumerated, than with any others, I have no doubt, since I had the advantage of comparing (August, 1850), with the assistance of M. De Koninck of Liege, the fossils from Kleyn Spauwen, Boom, and other Limburg localities, with those from Mayence, Alzey, Weinheim, and other Rhenish strata. Among the common Belgian and Rhenish shells which are identical, I may mention _Cassidaria depressa_, _Tritonium flandricum_ De Koninck, _Cerithium tricinctum_ Nyst, _Tornatella simulata_, _Rostellaria Sowerbyi_, _Nucula Deshaysiana_, _Corbula pisum_, and _Pectunculus terebratularis_. From these Upper Eocene deposits of the Rhine M. H. von Meyer has obtained a great number of characteristic fossil mammalia, such as _Palaeomaeryx medius_, _Hyotherium Meissneri_, _Tapirus Helveticus_, _Anthracotherium Alsaticum_, and others. The three first of these are species common to some of the lignite, or brown coal beds in Switzerland, commonly classed with the molasse, but of which the true age has not yet been distinctly made out. The fossils of the sandy beds of Eppelsheim, comprising bones of the Deinotherium, Mastodon, and other quadrupeds, are regarded by H. von Meyer as belonging to a mammiferous fauna quite distinct from that of the Mayence basin, and they are probably referable to the Miocene period.