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 bya 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 teenioglossate 
radula, whose formula is I: I: 4:1:21:1: 1 (or 
2:1:1:1: 2, Plate IIL, 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 Volutide (Plate III., Fig. 18) 
the process is carried a step further, and only the 
