INTRODUCTION. XXV 



the earlier writers. To other rejected species, where deemed 

 advisable, a more or less brief description has been appended, 

 that continental authors may at length definitively ascertain 

 what was actually intended by Montagu, Turton, their 

 predecessors and contemporaries. 



The distribution, in depth, of Mollusca in the British 

 seas, has been made the subject of numerous and minute 

 researches. Under each marine species described in this 

 work, full particulars of its bathymetrical range are usually 

 given, and instances illustrative of its habitat at different 

 and distant points. Those of our readers who would study 

 this interesting subject in all its bearings, should consult 

 the essay " On the Connection between the Distribution of 

 the existing Fauna and Flora of the British Isles, with the 

 Geological Changes that have affected their Area,'' con- 

 tained in the first volume of the " Memoirs of the Geologi- 

 cal Survey of Great Britain," and the " Report on the 

 Investigation of British Marine Zoology by means of the 

 Dredge,"" in the volume of " British Association Reports 

 for 1850." There is also a very interesting paper on this 

 subject detailing the results of extensive experience, by Mr. 

 M'Andrew, published in the "Transactions of the Literary 

 and Philosophical Society of Liverpool.'"' 



A few remarks will serve to explain the notes on bathy- 

 metrical range, appended to our descriptions of species. 

 Marine Mollusca are mentioned as inhabiting a depth of 

 water of so many fathoms, or as having been dredged from 

 such a depth. We need scarcely remind our readers that 

 the term fathom, in depth, is applied to a vertical measure 

 of six feet. It is employed in preference to other terms, 

 because it is in general use among sailors, and is the measure 

 commonly inscribed upon hydrographical charts. Marine 

 Mollusca are also mentioned as inhabiting different Zones 



d 



