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CHAPTER XL 



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 BatJianalia, and thus may be said to 

 constitute a Limnotrochoid group. We saw, further, that 

 the genera Paramelania and Byfhoceras, 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, 



