248 A MANUAL OF AMERICAN LAND SHELLS. 



Family HELICID^. 



HELIX, Lm. 



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

 have long been convinced of the necessity of recognizing among its 

 species numerous distinct genera. I had, however, uj) to the publica- 

 tion of Terr. Moll. U. S., V., eliminated those species only whose jaw 

 has no distinct ribs upon its anterior surface. The balance of the spe- 

 cies I retained, grouped as subgenera only. Before recognizing these 

 groups as distinct genera, I desired to wait until we can ascertain 

 whether generic characters can be found in the jaws and lingual den- 

 tition as well as in the shells. Convinced that characters cannot be 

 found in these organs or in the genitalia, I adopted in that work the 

 dismemberment of the genus, so much demanded by the number 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, TacJiea, Eiqyaryplia, 

 Arionta, and Aglaja have few, separated ribs, usually grouped near 

 the center of the jaw, leaving both extremities without ribs. Mesodon, 

 Triodopsis, and Polygyra have numerous, separated ribs, spread over 

 the whole of the jaw. Stenotrema \i2i8 numerous, stout, crowded ribs, 

 also spread over the whole surface of the jaw. The ribs are also nu- 

 merous, crowded, and similarly disposed in StroMla, Gonostotna, Dor- 

 casia, and Fruticicola, but they do not so deeply denticulate both mar- 

 gins, as in the genera mentioned above. All the above have a high 

 jaw. The following have a much lower jaw : Vallonia, with numerous 

 crowded ribs, slightly denticulating the margins, especially the lower 

 one; AcantJmiula, with similar ribs, but quite arched; Glyptostoma, 

 with still more numerous, separated ribs, deeply denticulating either 

 margin ; and Polygyrella, with more numerous ribs, and proportionally 

 much wider to its height than in any of the other North American sub- 

 genera. Thus there seems to be some distinctive subgeneric character 

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

 tions in some of the subgenera where the species are numerous j thus, 

 in Arionta I found numerous ribs in rujicincta, though the other species 

 have but few. The number, disposition, and size of the ribs vary 



