In several places, as at Woolwich on the Thames, at Newhaven in Sussex, and elsewhere, a mixture of marine and fresh-water testacea distinguishes this member of the series. Among the latter, _ Cyrena cuneiformis_ (see Fig. 216) and _Melania inquinata_ (see Fig. 217) are very common, as in beds of corresponding age in France. They clearly indicate points where rivers entered the Eocene sea. Usually there is a mixture of brackish, fresh-water, and marine shells, and sometimes, as at Woolwich, proofs of the river and the sea having successively prevailed on the same spot. At New Charlton, in the suburbs of Woolwich, Mr. de la Condamine discovered in 1849, and pointed out to me, a layer of sand associated with well-rounded flint pebbles in which numerous individuals of the _Cyrena tellinella_ were seen standing endwise with both their valves united, the siphonal extremity of each shell being uppermost, as would happen if the mollusks had died in their natural position. I have described[6] a bank of sandy mud, in the delta of the Alabama River at Mobile, on the borders of the Gulf of Mexico, where in 1846 I dug out at low tide specimens of living species of _Cyrena_ and of a _ Gnathodon,_ which were similarly placed with their shells erect, or in a posture which enables the animal to protrude its siphon upward, and draw in or reject water at pleasure. The water at Mobile is usually fresh, but sometimes brackish. At Woolwich a body of river-water must have flowed permanently into the sea where the _Cyrenæ_ lived, and they may have been killed suddenly by an influx of pure salt-water, which invaded the spot when the river was low, or when a subsidence of land took place. Traced in one direction, or eastward towards Herne Bay, the Woolwich beds assume more and more of a marine character; while in an opposite, or south-western direction, they become, as near Chelsea and other places, more fresh-water, and contain _Unio, Paludina,_ and layers of lignite, so that the land drained by the ancient river seems clearly to have been to the south-west of the present site of the metropolis.