THE MOLLUSCAN RADULA. 



125 



sets free from the portion corresponding to each row of teeth 

 a pair of substantial siliceous plates of rectangular outline 

 when seen en face. On each of these are seated the red- 

 brown cusps of the lateral teeth. The cusps are detached 

 from the plates by further boiling. The basal pieces of each 



5' 



Text-Figure 2. — A siliceous basal plate and the cusps carried by 

 it, isolated from the radula of Cryptobranchia concentrica by 

 the action of boiling nitric acid, ss' Surfaces of attachment of the 

 cusps and plate to one another. 



side of the radula^ though free from those of the other side, 

 are closely fused among themselves, leaving no trace of the 

 outlines of separate pieces. 



The teeth, after treatment Avith nitric acid, have a refrac- 

 tive index which closely approaches that of chloroform; 1*4 

 being a little lower in the case of A. virginea, and a 

 little higher in that of A. saccharin a, there is a corre- 

 sponding difference in the value of the specific gravity, which 

 was found to be 2*1 in the latter and 2*0 in the former species. 

 It is open to doubt, however, whether this difference is con- 

 stant, for the examples investigated were far from numerous. 



