PHYLLIVOEA. 
63 
In Limax carinatus^ and some allied species, we 
have this clearly exemplified, the changes from 
the typical form into that at the edge being very 
gradual, and showing every possible connecting link, 
fig. 21 . a — d. But this gradual progression is far 
from being the case in all species. For example, a 
sudden and abrupt change in form is to be seen 
in the fourth lateral tubercle of Zonites radiatulus^ 
fig. 22. c. This sudden change of form is owing to 
the absence of the connecting links, which a refer- 
ence to the progressive alteration in other species 
will readily supply. Wherever a straight transverse 
line is observable in the arrangement of the lateral 
teeth, all the teeth in that line are similarly formed, 
whether the right or left laterals are in the same 
line, as in Planorbis contortus^ fig. 18. or divaricate 
from each other at the central tooth upwards, as in 
Achatina acicida^ fig. 18. rf., or dovvrnwards, as in^/zcy- 
lus fluviatilis^ fig. 18.5. Wherever the curve presents 
great angularity, as in Zonites radiatulus, fig. 18. c., 
there we find a sudden change in the form of the 
teeth, while in like manner a gradual curve {Limax 
carinatus, fig. 18. g.') is the result of a gradually pro- 
gressive change in the form of the teeth, the degree 
of deviation from a straight line being exactly in 
proportion to the amount of change which takes 
place between the form of the central and edge teeth. 
The following table gives some idea of the 
number of teeth in a transverse or horizontal row, 
of a few species, together with the number of rows 
upon the tongue, and the whole number of teeth on 
that organ. 
