THE TANGANYIKA PROBLEM. 
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possessed a similar primitive type of alimentary canal. Or, 
in other words, it would appear that we have in these 
features a clear indication that the halolimnic mollusca 
are primitive as a whole. When, however, we try to 
push their relationship further, we find that owing to 
the present unsatisfactory state of our knowledge of 
the Prosobranchiate group of gastropods, it is by no means 
so simple an affair as one might suppose, even after 
having carefully examined the anatomy of each of the 
members of the halolimnic series to determine satisfactorily 
with what recognised groups their affinities really lie. 
Something of the nature of these difficulties will become 
apparent if we consider the following facts. In the first 
place, Typhobia and Nassopsis have been already said to 
belong to the Melaniadae by the conchologists, but from 
what we now know about the very wide morphological 
difference which distinguishes the organisation of Typhobia 
from that of Nassopsis , it is quite clear that both these 
organisms cannot possibly be regarded as belonging to the 
same family. If one of them is a Melania , the other is 
not. And, as a matter of fact, what is a Melania ? The 
genus Melania was founded by Lamarck upon the Mada- 
gascan species, Melania amarula , but as Bouvier rather 
plaintively points out, there is not one single anatomical 
feature which can be said to distinguish M. Amarula from 
Ceritkium vulgatum , a form which, according to existing 
classifications, is relegated to another family, the Cerithidce. 
Melanopsis , as Bouvier found, is as widely separated from 
the type of Melania in one direction as Nassopsis is from 
Typhobia in another ; and these few examples will, it is 
trusted, bring to light the fact that the so-called group, the 
Melaniadte, is no real group at all ; as it exists in the litera- 
ture of to-day it is an entirely heterogeneous assemblage 
