THE MOLLUSCS OF THE GHEAT AFRICAN LAKES. 171 



epitomised in the same tabular form I used while speaking 

 of Nyassa. From this table it will be clearly seen that if we 

 exclude the Halolimnic fauna altogether, the bathymetric dis- 

 tribution of the molluscs both in Nyassa and Tanganyika is 

 approximately the same. In both cases the fresh-water 

 molluscs are restricted to the sheltered, shallow portions of 

 the lakes. But directly we pass from the consideration of 

 the normal fauna to that of the Halolimnic forms the most 

 striking changes are at once observed. Instead of the Halo- 

 limnic molluscs being restricted to the shallow creeks and bays 

 about the coast, they swarm on the rough surf-swept rocks and 

 on the open beach. And what is more remarkable than this, 

 they extend in great profusion to the deepest portions of the 

 lake. Thus, dredging in water which varied in depth from 

 800 to 1200 feet, I always obtained plenty of Ty phobia, 

 Paramelania, Bathynalia, and Bythoceras among the 

 Gastropods, as well as the so-called Unio Burtoni among 

 the Lamellibranchiata; and how far these genera extended be- 

 yond these depths I cannot say, but they showed no signs 

 of dying out, but rather the reverse. On the lake floors which 

 were not so deep as this, from 200 to 300 feet below the sur- 

 face, but which were yet deep enough to have yielded nothing 

 by dredging in Nyassa, there was an abundance of Limno- 

 trochus, Syrnolopsis and Neothauma, together with 

 those varieties ofMelania which inhabit Tanganyika. It is 

 thus rendered apparent by these observations that the Halo- 

 limnic molluscs are all either surf-swept rock dwellers, or 

 entirely deep-water forms. Unfortunately we are as yet 

 entirely ignorant of the distribution of the molluscs in any of 

 the great lakes besides the two which I have named. But as 

 the normal fresh-water fauna of Nyassa and Tanganyika have 

 the same bathymetric distribution, it is probable that these 

 same genera inhabiting the remaining lakes which have not 

 yet been investigated will be found to be similarly disposed. 



It is thus apparent that the Halolimnic molluscs are com- 

 pletely dissociated from the normal fresh-water forms, along 

 with which they exist in Tanganyika, not only by their singular 



