270 
THE TANGANYIKA PROBLEM. 
of Prosobranchs, which have simply been lumped together 
through the still prevailing and pernicious habit of using 
conchological characters to distinguish species, and also 
because, as Bouvier remarks, a large number of forms, the 
wide anatomical differences between which are not apparent 
in their shells, have a fresh-water habit, and have, in con- 
sequence, been supposed to belong to a single morphological 
group. 
If we take Melania amarula as a typical Melania , the 
Typhobia group in Tanganyika has nothing to do with the 
Melaniadae at all. All the members of this group are at 
once and completely dissociated from such a form — (1) by 
the character of their radula: ; (2) by the gastric structures 
which I have described, and which neither M. amarula 
nor any of the other known members of the so-called 
Melania group possess ;* and (3) by a number of minor 
details, including the possession of an introvertible penis 
in the Tanganyika forms, the peculiar property of the 
Typhobia group, and of which there is no trace whatever 
in M. amarula. 
The radula of Typhobia and the radula of Bathanalia 
are remarkable, and are closely similar to the typical radula: 
of the genera Xenophora, S trombus and even Capulas. 
In the characters of their nervous system the members of 
the Typhobia group approach both Melania amarula and 
Cerithium , for they exhibit the shortening of the sub- 
intestinal cord, and the approximation of the left pleural 
and sub-intestinal ganglia, which is a characteristic of these, 
* Since the above was written an examination of some of the so-called Melanias from 
the East Indian Islands has shown that these, at any rate, possess a rudiment of the style 
sac, an observation which tends to show that divergent and old types have, in different 
regions, independently taken to the fresh water of the land as members of the secondary 
fresh-water series to which I have alluded in the second chapter of this work. The 
structure is also present in Turritella communis. 
