IV PREFACE. 



tive external likeness which may mask fundamental differences. 

 These considerations justify, I believe, the stress placed upon mere 

 system in this volume. The treatment of minor groups may be ob- 

 jected to as unduly minute; and it is true that most groups seem 

 over-divided. As my predecessors are responsible for'most of this, 

 I have been satisfied to reflect their labors faithfully. Those groups 

 having important structural characters I have considered generic; 

 grouping under these as subgenera and sections the various smaller 

 assemblages, which specialists find useful, but which are usually of 

 little systematic value, and not much utility to the general malacol- 

 ogist. These remarks imply no disrespect to the founders of this 

 multitude of groups. Their labors were necessary in pointing out 

 the differential features of Helices. They sought differences, for the 

 establishment of new groups ; the modern systematist seeks more 

 profound likenesses, in order to establish lines of descent. The 

 splitting of faunas into minute groups has taught us the compara- 

 tive value of characters, paving the way for more philosophical 

 study of the genealogy of faunas. The torch of analysis lights the 

 path for synthesis. 



It will, of course, be obvious that a general idea of the contents of 

 the principal divisions of Helicidre as here distinguished, must be 

 obtained before the geographic hypotheses can be rightly under- 

 stood. 



Ackuoivledgements and Thanks. That a large number of Helicoid 

 groups are made known anatomically in this work is primarily due 

 to the kindness and generosity of many conchologists who have sup- 

 plied living or alcoholic material for dissection ; and while it would 

 be impossible to name here all those who have thus assisted me with 

 specimens, notes on distribution, synonymy, etc., I must express 

 my obligations for material for investigation to W. G. Binney, John 

 Brazier, Alfred Caruana Gatto, Dr. J. C. Cox, Wm. H. Dall, Henry 

 Hemphill, J. B. Henderson, C. W. Johnson, O. von Mollendorff, 

 Morris Schick, Dr. Benj. Sharp, Dr. H. Simroth, Frederick Stearns, 

 Henry Suter and Rev. R. Boog Watson. A series of mounted rad- 

 ulie which I owe to Rev. Prof. H. M. Gwatkin, has enabled me to 

 illustrate the teeth of many interesting genera, among them Oxy- 

 chona, Macrocyclis, Albersia, Planispira, Entodina, Acavus and 

 others. My friend, Charles Hedley, of Sydney, has contributed not 

 a little to views both systematic and theoretical expressed herein, 

 but my main debt to him is for help more subtle than this. 



