252 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



HELIX, Lix. 



In common with all who have studied the Pfeifferian genus Helix, I am con- 

 vinced of the necessity of recognizing among its species numerous distinct 

 genera. I have, however, up to this time eliminated those species only whose 

 jaw has no distinct ribs upon its anterior surface. The balance of the species 

 I retained grouped as subgenera only. Before recognizing these groups as 

 distinct genera, I desired to wait until we can ascertain whether generic char- 

 acters can be found in the jaws and lingual dentition as well as in the shells. 

 Convinced that characters cannot be found in these organs or in the genitalia, 

 I now adopt the dismemberment of the genus so much demanded by the num- 

 ber of its species, founding the distinction on the shell alone. I shall discuss 

 the constancy of the jaw and lingual dentition under each group, as far as our 

 material will allow. In this place I will merely mention that in general terms 

 it may be said that Pomatia, Tachea, Euparypha, Arionta, and Aylaja have 

 few, separated ribs, usually grouped near the centre of the jaw, leaving both 

 extremities without ribs. Mesodon, Triodopsis, and Polygyra have numerous, 

 separated ribs spread over the whole of the jaw. Stenotrema has numerous 

 stout, crowded ribs also spread over the whole surface of the jaw. The ribs 

 are also numerous, crowded, and similarly disposed in Strobila, Gonostoma, 

 Dorcasia, and Fruticicola, but they do not so deeply denticulate both margins 

 as in the genera mentioned above. All the above have a high jaw. The fol- 

 lowing have a much lower jaw ; Vallonia, with numerous crowded ribs slightly 

 denticulating the margins, especially the lower one; AcantMnula, with similar 

 ribs, but quite arched ; Glyptostoma, with still more numerous, separated ribs, 

 deeply denticulating either margin ; and Polyyyrella, with more numerous ribs, 

 and proportionally much wider to its height than in any of the other North 

 American subgenera. Thus there seems to be some distinctive subgeneric 

 character to the jaw. It must, however, be borne in mind that there are 

 exceptions in some of the subgenera where the species are numerous ; thus, in 

 Arionta I found numerous ribs in nificincta, though the other species have but 

 few. The number, disposition, and size of the ribs vary within certain limits 

 in different individuals of the same species. I have repeatedly found this to be 

 the case. 



In regard to the generic value of the type of lingual dentition, I can only say 

 in general terms that within certain limits it may prove reliable. Here again, 

 however, we find the type of dentition inconstant when many species are 

 known. Thus in Arionta we find Townsendiana quite differing from the other 

 known species (see below). In Mesodon, also, I find two quite distinct types 

 of dentition, and under each genus I have pointed out the variation observed. 

 I am convinced that the presence or absence of side cusps to central and lateral 

 teeth is not a reliable subgeneric character. The same may be said of the side 

 cutting points. The marginal teeth offer more reliable characters. ' They are 



