THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 171 



the specimeu of Fauuus to which I have referred (PI. 15, 

 fig. 2). 



There is little or no trace of the spermatic groove in S. 

 zonata beyond the termination of the flap, but this groove 

 is very plainly indicated in this position in Faunus (PI. 15, 

 fig. 11, g, V.) and in Littorina (PI. 15, fig. 1, g. v.). 



In all essential details the female apparatus of zonatus 

 repeats that of the male, but there is neither an external 

 groove nor pouch. 



Comparative. 



In attempting to ascertain with what known raolluscan 

 forms the foregoing species are to be placed, it will be abun- 

 dantly apparent from the previous anatomical descriptions 

 that each bears little if any relation to the other ; and further 

 that from the broad features of their general anatomy the 

 existing conchological determinations of their affinities with 

 the Hydrobiidse must be at once dismissed. As regards the 

 so-called S. zonata, the general anatomical features of this 

 form, the gills, the position of the viscera, the nerves, and so 

 forth, all place it unquestionably among the simpler Naticoids ; 

 the only features which at first sight might militate against 

 such a determination of its phylogenetic relationships beino- 

 those presented by the radula and dentition. We have seen 

 that the dentition of S. zonata is in many ways identical with 

 that Melanio-planaxoid type which I have already described, 

 and that it only corresponds with that of any of the Nati- 

 coids in the minor features of the median tooth. It is there- 

 fore incumbent on us to ascertain whether this difference in 

 the radula is really an important feature, or whether it is 

 not wholly or partly due to the antiquity or the specialisation 

 of the Tanganyika form. But in order to obtain a clear con- 

 ception of this matter it is necessary to examine the radula 

 question from a more general point of view. I have long been 

 impressed by the possibility that the usually adopted methods 

 of estimating the gross relationships of these structures simply 



