in February, 1888, the task fell to me of continuing 

 the Monograph of the Helices left unfinished by the eminent Con- 

 chologist GEO. W. TRYON, it seemed to me desirable that the work 

 should be completed on essentially the same plan followed in 

 the two volumes of Helicoids already issued. A year's experience 

 has convinced me, however, that certain changes would increase the. 

 utility of the work to the naturalist ; and in the present volume 

 these modifications have been carried out. Chief among them are 

 the introduction of complete synonymy into the body of the work 

 (instead of reserving it for the index), and the preparation of fuller 

 descriptions, noticing every character shown by the specimens before 

 me of each species, a step which the critical, analytical methods of 

 modern Malacology has made necessary. In no group known to me 

 are species separated upon slighter differences than in the Helices. 

 In many subgenera not only are figures necessary for satisfactory 

 determination of the species, but descriptions extending to the most 

 minute and microscopic details of sculpture and form. 



Without entering at length upon the much-debated and profitless 

 question of what constitutes a species, the author may state that in 

 the present work any race of similar forms is regarded as a species 

 if it exhibits characters tangible enough to admit of definition and 

 recognition, and has not been shown to be actually united by recent 

 intermediate forms with allied races. Of course when the Tertiary 

 fauna becomes known, many of our distinct species will prove to be 

 united with others ; but for present purposes, it is enough that there 

 is a hiatus or break in the chain of forms which enables us to define 

 and separate a group of individuals from all other recent groups. 

 A variety differs from a species only in this : coalescence of its char- 

 acters with other forms actually occurs in some individuals. It fol- 

 lows that forms not very different may often be regarded as separate 



