49 



movements of the radula over the cartilage were not the 

 usual method of attack. 



The radular teeth (fig. 14) are, in accordance with 

 the Rachiglossan formula, three in number, one median 

 and two lateral. The median teeth, known as the central 

 or rachidian, are placed with their anterior margins 

 exactly transversely across the radula and possess a 

 number of similar denticles which will be referred to 

 again below. A thickened yellow band marks the 

 position of the tooth itself. All the teeth are fused to 

 the chitinous radular band. The lateral teeth are set 

 obliquely, and almost alternate in position with the 

 centrals. The inner end of a lateral tooth lies almost 

 opposite the base of a central tooth, whilst its outer end 

 is opposite the next posterior central. Furthermore, the 

 lateral teeth are not exactly on the same plane as the 

 centrals, and as the radula passes into the lingual groove 

 they are caused to rotate until their denticles are directed 

 towards the middle line. The denticles of the lateral 

 teeth are much larger than those of the centrals and vary 

 in shape, the outermost being by far the most powerful. 



On the whole, the radular teeth are very regular and 

 characteristic in the gastropoda, and are commonly used 

 for purposes of classification. The teeth of Buccinum 

 undatum, however, make a very striking exception to 

 this rule, since the number of denticles on both the 

 central and lateral teeth varies in an extraordinary 

 manner, giving an excellent example of meristic varia- 

 tion. The three first radulas taken from Port Erin 

 whelks varied in number of denticles as follows : — 

 Lateral teeth : — 4. 4. 4 

 Central teeth : — 6. 7. 8. 



Bateson records that from 27 specimens the varia- 

 tions were as follows : — 



