198 PROCEEDINGS OF THE ACADEMY OP 



can be found in the jaws and lingual dentition as well as in the 

 shells. I shall discuss the constancy of these characters 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 Poma. 

 iia, Tachea, Enparypha, Arionta, and Aglaja have few, separated 

 ribs, usually grouped near the centre of the jaw, leaving both ex- 

 tremities without ribs. Mesodon, Triodopsis, and Polygyra have 

 numerous, separated ribs spread over the whole of the jaw. Ste- 

 notrema 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 following have a much lower jaw : Vallonia, with nu- 

 merous crowded ribs slightly denticulating the margins, especially 

 the lower one ; Acanthinula, with similar ribs, but quite arched ; 

 Glyptostoma, with still more numerous, separated ribs, deeply den- 

 ticulating either margin; and Polygyrella, 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, how- 

 ever, 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 ruficincta, 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 subgeneric value of the type of lingual denti- 

 tion, 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 (pi. XVIII., fig. 8) quite differing 

 from the other known species (see below). In Mesodon, also (pi. 

 XII.), I find two quite distinct types of dentition, and under each 

 subgenus I have pointed out the variation observed. I am con- 

 vinced that the presence or absence of side cusps to central and 

 lateral teeth is not a reliable subgeneric character. The marginal 

 teeth offer more reliable characters. They are very peculiar in 

 Vallonia and Sb'obila, in being very low and wide, and having 

 numerous cutting points, quite resembling those of Pupa. 



