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CHAPTER XI. 
THE FAUNA OF LAKE TANGANYIKA. 
OF THE NATURE OF THE HALOLIMNIC GASTROPODS. 
Turning from the foregoing survey of the structure 
presented by the halolimnic gastropods to the question of 
what relationship they bear to the different types of pro- 
sobranchiate molluscan organisation with which naturalists 
are acquainted, it is necessary, in the first place, to sort out 
the halolimnic forms themselves into separate groups, for 
it will have become apparent that some of these exhibit more 
or less close affinities one with another, while others do not. 
Thus the genera Typhobia and Bathanalia are unques- 
tionably close allies, and we may consequently speak of 
them as characterising a Typhobian group. Limnotrochus 
and Chytra show closer affinities with each other, than 
with Typhobia and Bathanalia , and thus may be said to 
constitute a Limnotrochoid group. We saw, further, that 
the genera Paramelania and Bythoceras . although they 
bore certain relationships to the members of both the above 
groups, were quite distinct from either of them, and, conse- 
quently, these genera form a third or Paramelanian group. 
Further, we found that Tang any icia, although it bore a 
certain resemblance to all the preceding forms, was quite 
distinct from any of them, and forms in itself a fourth, 
