OF THE LEVEL OF THE SEA. 87 



It is quite obvious that no single movement of elevation 

 or of depression can account for the phenomena presented 

 by these beds of marine or terrestrial origin. We find in- 

 disputable traces of several ; some of them separated by long 

 intervals of time. But however numerous the changes, 

 however vast the intervals, they form but one page in the 

 voluminous history of the earth which geology unfolds. 

 That page has been but just opened, and the few feeble cha- 

 racters inscribed upon it by so early a labourer, must be 

 necessarily imperfect. 



In drawing up the following catalogues, I have adopted 

 the arrangement of Lamarck as the one most generally 

 known, only adding genera from other authors when 

 necessary to include new species. I am aware that, by ad- 

 hering perhaps too closely to this rule, I have in some in- 

 stances separated species which ought to have been classed 

 together. But my object being altogether geological, viz. 

 the comparison of the ancient and modern deposits, I was 

 anxious to make no change which could be avoided. I 

 have, however, separated the land and fresh water shells 

 from those of the sea, because it is only amongst the latter 

 class that the comparison in the present instance can be of 

 any value. The marine catalogue contains all the British 

 newer pliocene shells hitherto discovered ; but as a very 

 large proportion of them occur in the basin of the Clyde, 

 and as the localities of those which do not are marked, their 

 insertion cannot be productive of mistakes. With the land 

 and fresh water shells, the case is different ; but few of the 

 fossil ones have come under my notice, or been found in the 

 districts to which my observations have been chiefly con- 

 fined, and none which differ from the known species. I have 

 inserted them therefore with some hesitation, for the purpose 

 of rendering the catalogues more complete. 



