THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 167 



How exactly this view accords with the results of the 

 anatomical comparison just given will be at once apparent. 

 And in conclusion I may add that, when I arrived at the 

 views above stated I was ignorant of Pelseneer's work, and of 

 the conclusion at which he had arrived. 



Spekia zonata. 



The shell of this remarkable mollusc is represented in PL 18.. 

 fig. 1, and it is certainly most curious that no attention has 

 been drawn by any of the conchologists to the extremely nati- 

 coid chai'acter which it presents, for the shell of this species is 

 so completely similar to that of numerous fossil naticoid forms 

 that, had it appeared fossilised instead of having been found 

 living in a great fresh-water lake, there is not the slightest 

 doubt that it would have been placed in one of the numerous 

 fossil genera which are supposed to group themselves about 

 the living Naticas. The oblique aperture, and the tendency 

 of the outer wall of the mouth to be continued as a cup-shaped 

 ring round the bases of the older shells, are exactly what is 

 observed in many fossil Naticas ; while the presence of a very 

 pronounced umbilical opening, which is more or less filled up 

 with a deposit of callous substance, are features which are 

 generally regarded as almost diagnostic of naticoid shells. 



The external appearance of this form is superficially similar 

 to that of T. rufofilosa. The foot is rather less broad, and 

 the snout is not so much pigmented ; but, apart from the 

 naticoid appearance of the shell, it is only in the internal 

 anatomy that we begin to appreciate the wide morphological 

 differences which exist between these forms. 



In S. zonata the buccal mass is well developed, and the 

 radular sac is conspicuous, but not of any considerable length 

 (PI. 18, fig. 6). There are two very strong muscles attaching 

 the buccal mass to the body-wall, and the salivary glands are 

 long and simple. The radular dentition is characteristic, and 

 very strongly developed. A single row of teeth is represented 

 in PI. 18, fig. 3. It will be at once seen that the characters of 



