VII. NOMENCLATURE. 



In preparing the generic nomenclature of the Lymnaeas, constant 

 reference has been made to Dr. W. H. Dall's recent work on the Land 

 and Fresh-Water Mollusks of Alaska, and his conclusions have been 

 accepted, in the main, and incorporated in the present work. So far 

 as specific nomenclature is concerned, the ruling recognized by Pilsbry 

 and other recent zoologists, of "Once a synonym, always a synonym," 

 is the only safe and satisfactory disposition of duplicated names, even 

 though the species are here placed in different genera. 



Regarding specific limitations, a wide difference of opinion exists. 

 Dr. Pilsbry well says : "The conception of species in such sedentary 

 animals as snails is far from simple. A 'species' comprises a multitude 

 of colonies or communities which at any one time are isolated one from 

 the other by the existing topographic and other surface features of 

 the country. This is and always has been the case, even with the 

 common, widespread forms of the more level part of the country ; but 

 the colonies there have always been subject to frequent mixture with 

 their neighboring colonies, by the ever slightly fluctuating conditions 

 of woodland and local moisture, so that their network over the country 

 has been here and there made practically complete within comparatively 

 short periods. As a consequence, we have in many cases no tangible 

 difference between individuals from colonies hundreds of miles apart." 



The above paragraph, while relating to land mollusks, seems to 

 apply equally as well to the Lyrrmaeidae, although the degree of differ- 

 entiation is manifestly not as great among the fresh-water pulmonates 

 as in the land pulmonates. The same rule, however, holds true for 

 both. Land shells are more often differentiated by isolation than are 

 the fresh-water pulmonates, but it is true, nevertheless, that isolation 

 has played an important part in species formation among the Lymnaeas. 



Great care has been used in determining the specific limits of the 

 Lymnaeas herein recorded. In nearly all cases the types have been 

 examined and no name has been placed in the synonomy unless there 

 were valid reasons for considering it a synonym. It may be thought 

 that too much liberality has been shown in thus recognizing many old 

 species long considered synonyms by Binney, Tryon, Dall and other 

 competent malacologists, and likewise the addition of such a large 



