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 sonata. 
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 zonala 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 
