PREFATORY REMARKS. ix 



in order to place on record that the several islands and archipe- 

 lagos (with the exception of the Azores) which are treated of in 

 this volume have been visited personally by myself. Neverthe- 

 less I should hardly have been inclined to undertake so serious 

 a task as the critical examination of the characters and habitats 

 of so many species, had not the bequeathment to me by Mr. 

 Lowe of his extensive conchological collections (to be distributed 

 to various Museums, though with power to reserve for my own 

 use whatever types I might require) thrown on to my hands a 

 mass of material so unexpected that, in order to do it full jus- 

 tice, I felt that it would be absolutely necessary to treat the 

 whole subject afresh, and to revise (so far as was practicable) 

 every form which has hitherto been published from the island- 

 groups to which the present memoir has reference. 



I will merely add that this Treatise is not intended to be a 

 Monograph, but rather a critical enumeration of all the forms 

 which have been recorded, up to the present date, in the several 

 Atlantic archipelagos ; nevertheless in most cases I have given 

 diagnostic remarks which it is hoped will be found useful, — if 

 not in every instance actually to identify the species, at any 

 rate to supplement the published descriptions of them, and to 

 point out more particularly in what they differ from their im- 

 mediate allies. And since I have the firmest conviction that 

 the question of habitat is even more important (if possible) in 

 a professedly geographical catalogue than elsewhere, I have 

 spared no labour in sifting the evidence for the exact localities 

 (in those instances where I have not been able to vouch for 

 them by personal observation), and have frequently preferred 

 to omit the latter altogether than run the risk of perpetuating 

 confusion by placing upon record what there is every reason to 

 suspect is not strictly accurate. This being the case, I have 

 been less anxious to erect new species than to clear up difficul- 

 ties concerning the old ones, and have always therefore avoided 

 doing so except in instances where the characters were well de- 

 fined and it seemed positively essential that the additional 

 forms should not be omitted from the list. Indeed, although 

 the mere titles of a few others have of necessity been altered, the 

 following twenty-nine are the only actual novelties which I 

 have considered it necessary to characterize : — 



