210 PROCEEDINGS OF THE ACADEMY OF 
go, I cannot find it. The outer laterals, however, in most of the. 
species have a much more developed side cusp than the inner 
laterals, bearing a well-developed cutting point (see pl. XI., fig. . 
6, d), but not all the species, as H. exoleta for instance, as shown 
in pl. XI., fig. 7, d, has no well developed side cusp and cutting 
point on its outer laterals, nor does it appear except on the de- 
cided marginals. It is the same in H. Sayit. 
I find also variation in the manner of passing from the lateral 
to the marginal teeth among the species of this first group of Me-: 
sodon. In H. exoleta (pl. XI., fig. 7, 14) the cutting point remains 
the same, and also in Sayit, profunda, Wetherbyi, and Mitchel- 
liana, but in elevata (pl. XII., fig. 1, 18) the transition teeth are 
characterized by the bifurcation of the large cutting point; the 
same occurs in albolabris, multilineata, Roémeri, Columbiana (pl. 
XIT., fig. 2) and devia, and the rest of the group. 
The general character of the teeth in this section of Mesodon is 
about the same as I have described above for Triodopsis (p. 260). 
It will be noticed, however, that the marginals (as in H. exoleta 
and Wetherbyz) do not always have their cutting points bifid. 
The other type of dentition in the subgenus Mesodon is shared 
by #. thyroides (pl. XVIIL., fig. 2), clausa (pl. XII., fig. 7), and 
Wheatley (pl. XVIIL., fig. 1). The centrals and first laterals have 
subobsolete side cusps without cutting points, the outer laterals 
have no side cusp, but retain the type of the first laterals, they are 
much longer, narrower, and have one extremely long, oblique, 
stout, bluntly pointed cutting point, reaching far beyond the 
lower margin of the base of attachment. These outer laterals 
pass gradually into the marginals, which retain their general form 
but have a less developed reflection, and much more proportion- 
ally developed cutting point, sometimes bifid in the extreme mar- 
ginals (pl. XVIII., fig. 2,54), and usually with a small side cutting 
point. . 
As in all the subgenera of Helix, the marginal teeth of MZesodon 
show great variation in their denticulation, even in most cases on 
the same membrane. H. Clarki (pl. XI., fig. 6) has the marginals 
with cutting points much blunter and broader in some parts than 
in the others. | 
The study of the dentition of Mesodon shows that we must be 
prepared to find considerable variation in the character of the 
teeth of any subgenus. The peculiar outer lateral teeth and mar- 
