XXIV INTRODUCTION. 



of each individual informant of an author, has been estima- 

 ted upon the opinion entertained by surviving contem- 

 poraries, of his veracity and caution ; the several shades 

 of difference between an author"'s positive declaration, that 

 he himself has dredged the species in a living state, and 

 the mere picking up, by some unlearned friend of his, 

 of single valves upon the beach, are duly taken into our 

 estimate ; the competency for the determination of species, 

 at the 'date of publication, of parties who have re-admitted 

 such into their local catalogues, has been, as far as possible, 

 scrutinized ; the improbability of a Boreal species being 

 discovered on our Southern shores, or of a tropical one upon 

 our Northern coast, the unlikelihood of a known littoral 

 shell being dredged in deep water at many miles from the 

 land, has been poised in the balance of conflicting testi- 

 monies : in fine, an almost unanimous verdict from the 

 chief living writers upon British Oonchology, and the most 

 zealous and trustworthy collectors, has preluded and sanc- 

 tioned our expulsion of any species which has ever been 

 improperly regarded as a veritable inhabitant of the soil 

 or waters of our Islands. 



Were we to preface such species with a circumstantial 

 detail of the evidences upon which our conclusions have 

 been grounded, a larger portion of our work would be 

 occupied by the spurious than by the genuine British Tes- 

 tacea. With a similar view of economising space, without 

 the omission of any matter important to Conchology, those 

 of the former, which are universally known, are simply 

 defined by a synonomy based upon the re-examination of 

 the types (or where that was impracticable, careful study 

 of the published descriptions), and not merely founded upon 

 the surmise that the British writer had correctly divined 

 the previous names bestowed upon them by Linnseus and 



