258 
THE TANGANYIKA PROBLEM. 
long and simple. T. he radular dentition is characteristic, 
and very strongly developed. A single row ol teeth is 
represented on page 257* It will be at once seen that the 
characters of these teeth are highly suggestive of those of 
a number of well-known forms. In gross detail the radular 
dentition is very similar to that occurring in various forms 
of Anchylotus figured by Troschel. The predominant 
denticle on the admedian tooth is well developed in zonata, 
as indeed it is in a very large number of dissimilar forms ; 
Fig. 41. — Nervous system of Spekia zonata. a. from above ; b. from 
the side ; b.g. buccal ganglion ; c.g. cerebral ganglion ; sup. int. g. 
supra intestinal ganglion ; p.g. pedal ganglion. 
and an exactly analogous and widely prevalent feature is 
presented in the difference of size and character between 
the denticles on the heads of the outer and inner lateral 
teeth. Perhaps, however, the most notable feature which 
the radula presents is the peculiar structure of the median 
tooth. The outer surface of this tooth is concave, like the 
median tooth in Anchylotus , Thiara , Melania brevis (Dorb.), 
and Melanopsis. But it differs from all these forms in 
having no predominant median denticle, there being instead 
two lateral predominant denticles and a median concavity. 
The only forms which appear to possess this peculiarity of 
