326 PALEONTOLOGY. 



lector unmistakeable features of the marine origin of the strata con- 

 taining them. 



Subsequent researches, aided by the refined conckology of modern 

 science, have established the truth of Hunter's conclusion. 



All the shells of the London clay which forms the bottom of the 

 tract through which the major part of the Thames flows, are of marine 

 species, and most of them extinct. In the superficial gravel have been 

 found fluviatile shells, most of them of recent species, with the remains 

 of elephant, rhinoceros, hippopotamus, and other large terrestrial 

 quadrupeds. 



The following remarks show how closely Hunter had studied his 

 fossil shells with a view to geological deductions. 



" Parts of sea-animals as were capable of being preserved till fos- 

 silized, such as shells, must have often lain long at the bottom of the 

 sea before the formation of the surrounding medium took place ; this 

 is plainly shown by the Pholas having eaten into them, which could not 

 have been done but when [they were] lying at the bottom of the sea. 



" Many fossil shells are covered with shells of another kind ; but this 

 may have taken place while the animal was alive in them ; as we often 

 see the same thing in recent shells. Pmt we often find that the shell 

 has become a mould, and afterwards the shell has been dissolved, and 

 only the cast left, and on this cast we shall find the shells of worms, 

 and the holes of the Pholas, so that the cast lay at the bottom of the 

 sea after the shell has been separated from it or destroyed. 



" Many shells are bruised, and have been afterwards filled with 

 matter, which also shows that they have lain some time at the bottom 

 of the sea, and that heavy bodies have been formed, and put or fallen 

 into motion. 



" Many have lain so long at the bottom of the sea as to have their 

 cavities filled with matter, and afterwards to have the shell entirely 

 destroyed, so that nothing but the cast remains, and upon this cast 

 living shell-fish have fastened themselves, similar to their fixing upon 

 any other stone in such situation ; all of which could never have been 

 done if the whole had not lain at the bottom of the sea for a con- 

 siderable time. 



" Many shells have lain at the bottom of the sea, where the water 

 has been agitated so much as to make them roll upon one another, or 

 other substances, by which they have been smoothed, some of which 

 have been afterwards enclosed in stone, <fcc. 



" Many shells would appear to have been lined with stone, and then 

 the cavity filled up with sand. Many have been encased with stone 

 and filled with the same, afterwards the shell has been destroyed, and 

 left the cast in the stone almost loose. 



