THE TANGANYIKA PROBLEM. 257 



the extremely naticoid character which it presents, for the 

 shell of this species is so completely similar to that of 

 numerous fossil naticoid forms that, had it appeared fos- 

 silised 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 



Fig. 40. — Lingual dentition of Spekia zonata, 



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 Tanganyicia 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 Spekia zonata the buccal mass is well developed, and 

 the radular sac is conspicuous, but not of any considerable 

 length. There are two very strong muscles attaching the 

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



17 



