6 TEST ACE A ATLANTIC A. 



genera in the Azorean archipelago, Mr. Tristram adds : ' There is, 

 however, one singular hiatus in the molluscous fauna. Though 

 there are abundant streams, springs, and lakes, presenting the 

 most favom'able conditions for their existence, not a single repre- 

 sentative of the Pulmobranchiate Mollusca has yet been disco- 

 vered. These are to be found in every other ^portion of the 

 globe. Not an island in the Pacific, not even Grreenland and 

 Iceland which are beyond the usual range of the Pulmonifera, 

 are ivithont representatives of this class ; yet in the Azores no 

 species of the world-wide genera of Limncea, Physa, Ancylus, 

 Neritina, Cyclas, or Gyrcena has yet been found.' Now to this, 

 again, I really cannot subscribe ; for, in point of fact, what do 

 we know about ' every other portion of the globe,' and of every 

 ' island in the Pacific ' ? In all probability we should find plenty 

 of instances in which the aquatic forms are wanting ; for, even to 

 come nearer home than the Pacific, the most remote and isolated 

 spot I have hitherto had an opportunity of exploring, namely 

 St. Helena, happens to be in precisely the same predicament as 

 the Azores. There are streams and tanks in the interior of that 

 island, in profusion, trickling rocks, waterfalls, and pools ; and 

 yet not a single freshwater species has occurred (beyond a 

 Succinea, which lives as well out of the moisture as in it, and the 

 modus Vivendi of which may well be paralleled by that of the 

 Hydrocoina gutta at the Azores). And so literally true is tliis, 

 that the same hiatus is equally observable in the Coleoptera, — 

 the Hydradephagous groups of which are altogether absent. 

 Moreover it seems far from unlikely that a similar deficiency may 

 be indicated in the Sandwich Islands ; at any rate it appears to 

 be so as regards the water-loving forms of the Coleoptera, — for 

 the Rev. T. Blackburn, wiiting lately from Honolulu, says (^vide 

 ' Ent. Month. ^lag.' xiii. 228) ' Notwithstanding the frequent 

 use of the water-net, I have not yet seen a single species of 

 Hyd ra depjhaga .' 



Perhaps a word or two may be desirable, before I conclude, 

 as regards the various hcdjitats which are cited in the present 

 section. Tlu'oughout the other portions of this volume the ma- 

 jority of the localities are added from my own personal observa- 

 tions ; and in the generality of the instances where that is not 

 the case, I have had abundant means for testing their accuracy. 

 The Azores, however, are to me a terra incognita ; and I have 

 been compelled therefore to rely, almost exclusively, on the pass- 

 ing remarks of MM. Morelet and Drouet. The question conse- 

 quently arises, where extreme precision is absolutely essential, 

 how far vague and general terms, such as are too often employed 

 with a looseness which is self-evident, can be trusted. Where 

 the actual islands are mentioned bv name, it would never occur 



