FOSSIL INDUSI^. 99 



of a small spiral univalve, (Paludina viridis,) fixed to the 

 outside of this tubular case of a larva of the genus Phry- 

 ganea. See Lyell's Principles of Geology, 3d edit. vol. iv. 

 p. 100. It is impossible to conceive how strata like these, 

 extended over large tracts of country, and laid one above 

 another, with beds of naarl and clay between them, should 

 have contained the coverings of such multitudes of aquatic 

 animals, by any other process than that of gradual accumu- 

 lation during a long series of years. 



In the case of deposites formed in estuaries, the admixture 

 and alternation of the remains of fluviatile and lacustrine 

 shells with marine Exuvia?, indicate conditions analogous 

 to those under which we observe the inhabitants both 

 of the sea and rivers existing together in brackish water 

 near the Deltas of the Nile,* and other great rivers. Thus, 

 we find a stratum of oyster shells, that indicate the presence 

 either of salt or of brackish water, interposed between lime- 

 stone strata filled with fresh-water shells among the Purbeck 

 formations ; so also in the sands and clays of the Wealden 

 formation of Tilgate forest, we have fresh-water and lacus- 

 trine shells intermixed with remains of large terrestrial rep- 

 tiles, e. g. Megalosaurus, Iguanodon, and Hylasosaurus ; 

 with these we find also the bones of the marine reptiles 

 Plesiosaurus, and from this admixture we infer that the 

 former were drifted from the land into an estuary which 

 the Plesiosaurus also having entered from the sea, left its 

 bones in this common receptacle of the animal and mineral 

 exuviae of some not far distant land.f 



Another condition of organic remains is that of which a 

 well known example occurs in the oolitic slate of Stones- 

 field, near Oxford. At this place a single bed of calcareous 



* See Maddcu's Travels in Egypt, vol. ii. p. 171.175. 



+ For the detailed iiistory of the organic remains of the Wealden forma- 

 tion, see Mr. Maiilell's highly instructive and accurate volumes on the geo- 

 logy of Sussex. 



