198 PROCEEDINGS OF THE ACADEMY OF 
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- 
tia, Tachea, Huparypha, Arionia, 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. Sée- 
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 Strobéla, 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 Arzonta, 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 (pl. X VIII., fig. 8) quite differing 
from the other known species (see below). In Mesodon, also (pl. 
XIT.), I find two quite distinct types of dentition, and under each 
subgenus I have pointed out the variation observed. JI 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 Strobila, in being very low and wide, and having 
numerous cutting points, quite resembling those of Pupa. 
