126 THE LIFE OF THE MOLLUSC A 



median tooth is left. In the Cones (Plate III., 

 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 



