EVOLUTION 125 



The radula (Plate III., Figs. 11-22), which is 

 characteristic of the phylum, exhibits consistent pro- 

 gress as one passes from the lower to the higher 

 groups. In the older families, especially the Rhipido- 

 glossa, the teeth are weak, but numerous ; indeed, 

 there may be as many as 300 or more in each trans- 

 verse row. The most archaic member of the group, 

 Pleurotomaria, shows the most primitive type and 

 indicates that it has been derived from an earlier 

 one, in which all the teeth in a transverse row were 

 similar. Now, in Pleurotomaria and the more primi- 

 tive Rhipidoglossa, three longitudinal tracts on either 

 side of the median tooth are distinguishable, but in 

 the higher members of this division it is found that 

 the several teeth in one tract are replaced by a single 

 large one, which generally retains sufficient traces of 

 the individuals it replaces to suggest that it repre- 

 sents the fusion of a series. Similar cases also occur 

 in molluscs yet higher in the scale. This fusion, 

 extending to all the tracts, results in the tasnioglossate 

 radula, whose formula is 1 : 1 : 1 : 1 : 1 : 1 : 1 (or 

 2:1:1:1:2, Plate III., Fig. 15). By further 

 fusion, or, rather, as would seem to be more probably 

 the case, by the abortion of the outer rows of laterals, 

 is derived the type of radula met with in the Rhachi- 

 glossa and more primitive of the Toxoglossa, whose 

 formula is 1 : 1 : 1 (Plate III., Fig. 17). In Harpa, 

 Marginella, and the Volutidse (Plate III., Fig. 18) 

 the process is carried a step further, and only the 



