126 THE LIFE OF THE MOLLUSCA 
median tooth is left. In the Cones (Plate IIL, 
Fig. 19), on the other hand, it is the median row 
that disappears, leaving the double row of barbed 
laterals. Of course this line of development is not 
exemplified in all. Numerous cases of apparent 
exception occur among families that lie off the main 
branch, or that in this respect retain archaic charac- 
ters. Such are Epitonium (Plate III., Fig. 14), Ian- 
thina, etc. 
The Opisthobranchia offer every variety of radula 
in their ranks from the uniform multiserial to the 
single row of median teeth in Elysia, and much 
further research will be necessary ere a solution of 
this diversity is found. 
The Pulmonata branched off early, and have 
retained a primitive form of radula, such modifica- 
tions as there are being principally in the number, 
and form of the cusps on the individual _teeth 
(Plate III., Fig. 21). These modifications proceed 
from the median line outwards, so that an inspection 
of the marginal teeth, or those of the embryo, gives 
a clue in many cases to the ancestral radula. Only 
one genus, Polita, shows an appreciable diminution 
in the number of teeth, for whereas the Common 
Garden Snail (Helix aspersa) has about fifty-five _ 
teeth on either side of the median in each transverse 
row, this number is reduced in Polita to at times 
as few as twelve. 
The higher Cephalopoda (Plate III., Fig. 20) have 
