INTRODUCTION, 



In closing this volume, which I now do with my fourth Memoir, I 

 look back with peculiar pleasure on the hours employed in investiga- 

 ting that family (the Naiades) which has been my chief study in 

 zoology. Induced at first to the study of conchology by the desire of 

 promoting my knowledge of geology, which recent discoveries prove 

 to be almost a blank sheet without the chart of its sister science, I 

 found myself launched into a new and delightful ocean, culling and 

 choosing among nature's choicest and most favoured works. The 

 geologists of the new schools regard shells as indicating, in the most 

 positive and direct manner, nearly all the strata above the primitive, 

 from the ' Lowest Fossiliferous Group' of De la Beche, to the high- 

 est of the series, the ' Modern Group,' (the ' Recent Period' of Lyell). 

 Between the shells of all these strata and those now living there is a 

 connection exciting almost a painful interest ; for although we find 

 recent deposits two thousand feet above the level of the sea,* contain- 

 ing species now inhabiting the neighbouring sea, with very few excep- 

 tions we always find, as we descend through the series, that the exist- 

 ing species and even genera gradually cease, until scarcely an indivi- 

 dual can be recognized to have its living analogue. 



* Principles of Geology, Vol. III. p. 126. 



