CLASSIFICATION OF HELICES. XXV11 



or modified by these influences, while internal organs are much less 

 directly acted upon, and lag behind in the process of transforma- 

 tion. For this reason, specific characters as well as those of sections 

 or subgenera are mainly drawn from the shell, while generic features 

 are usually found in the dentition, jaw and genitalia. As a rule, 

 these internal organs in any genus, show a far smaller range of 

 variation than the shells. In this connection it may be noted that 

 the appendages or organs of the foot (such as operculum, mucus 

 glands, pedal grooves, etc.) show much less variation in any natural 

 genus or family than the mantle organs (shell, mantle lobes, etc.). 



The generally acknowledged facts recited above, with the conclu- 

 sions reached regarding the taxonomic value of the shell (page vii), 

 the jaw (p. xi) and the teeth of the radula (p. xiii), have as their 

 logical outcome, caused us to form a classification of the land snails 

 based upon all the main features of the auimal economy, special de- 

 pendence being placed now upon one, now upon another system of 

 organs. Former arrangements of the genera based upon one or two 

 organs, must sooner or later be abandoned. Neither jaw, teeth or 

 shell, taken singly, prove to be sufficiently stable, nor is v. Iheriug's 

 primary division of Pulmonata into Micronoten (small-mantled, such 

 as Helix, Limax, Pupa), and Meganoten (large-mantled, as Vagin- 

 ulus, Philomycus), any more natural. 



In the opinion of the writer, a natural classification of Pulmon- 

 ates should be based upon : 



Organs of protection (shell, mantle, integument of body). 

 Organs of locomotion (foot with pedal-grooves, tail gland, etc.). 

 Organs of reproduction (genitalia, comparative size of eggs, etc.). 

 Organs of nutrition (jaw and teeth, intestinal tract, kidney). 

 Nervous system (including sense-organs such as tentacles, etc.). 

 Muscle system. 



In applying this scheme to the Helices, I have not attempted to 

 use characters of the nervous system, partly because neither the re- 

 quisite time or space is at my command, partly because other organs 

 promised results of more immediate utility. The other organic 

 systems named I have tried to study impartially. Although the 

 foundation of this system throughout rests upon comparative anato- 

 my, I have been influenced in some cases, where anatomical data are 

 wanting or insufficient, by the facts of geographical distribution ; 

 but this class of facts I have purposely held subordinate to anatom- 



