v.] ' INTRODUCTION. Ixxxix 



Isles, with reference to other countries and to our upper 

 tertiary deposits, will be subjoined to this volume; and 

 I propose to give in the succeeding volumes similar 

 tables to show also the distribution of our marine 

 Testacea. 



Origin of British Mollusca. — As regards the '' history" 

 or proximate origin of the British Mollusca, I fully agree 

 with Forbes and Hanley, w4io stated in the Introduction 

 to their work (vol. i. p. xxxv), that " the true source of 

 our Molluscan fauna was first manifested by the assem- 

 blage of Testacea preserved in the deposit called Coralline 

 Crag," although my investigation of the Crag shells has 

 not led me to form the same conclusion that they did, 

 viz. that most of these ancestors of our living shell-fish 

 are " of those forms which we regard as Southern types." 

 The opportunities afforded by a study of the Crag strata 

 are far superior to any, that we at present possess, for the 

 investigation of our marine Mollusca. We can explore 

 the ancient sea-bottom for many miles on dry land, and 

 as leisurely as if the bed of the present ocean were un- 

 covered and laid dry by some violent convulsion of 

 nature ; and this examination can be extended not only 

 superficially, but also by making sections of the bed to 

 a depth of thirty feet, so as to have the whole of its con- 

 tents exposed to view. In attempting a similar explora- 

 tion of the present sea-bottom, we are only able, at con- 

 siderable expense, with some personal discomfort, and 

 in such weather as we too frequently meet with in this 

 climate, to scrape up with the dredge a few bagfuls of 

 sand or mud mixed with shells ; nor can we hope to 

 examine in this way more than a very few inches in 

 depth. Many deep-burrowing shell-fish altogether escape 

 our observation, or are only procured by chance. 



In order to ascertain the exact nature of the relations 



