THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 167 



When comparatively examined, the observations which I 

 was enabled to make respecting the bathymetric distribution 

 of the molluscs in the lakes will be seen to show quite as 

 clearly as the facts of their geographical distribution that the 

 Halolimnic series is something entirely distinct from the 

 normal fresh-water population of the lake. Through the kind- 

 ness of Sir Harry Johnston I was enabled to attach myself to 

 one of the Nyassa gunboats, and thus to become acquainted 

 with the facts of molluscan distribution in Nyassa before I 

 reached Tanganyika on my way north ; and the observations 

 made on this voyage have been of the utmost value as material 

 for comparison with what I subsequently saw. 



Nyassa is a relatively narrow lake of great and unknown 

 depth ; soundings of 300 fathoms, no bottom, having been 

 obtained throughout a great proportion of its area. In total 

 length it covers some 340 miles, and it varies from 20 to 40 

 miles across. The shores are of the most varied description, 

 steep and precipitous in some places, in others bounded by 

 extensive flats. The lake has a free outlet down the Shire 

 River and the Murchison cataracts, the water being conse- 

 quently clean and fresh. Owing to the lake's great extent, 

 the shores are often swept by a heavy surf, and the fairly 

 strong currents which are observed are probably produced by 

 the trade winds, for they seem generally on the surface to set 

 from south to north. The fauna of such a lake is exposed to 

 the same conditions as those to which it would be on an open 

 oceanic coast. The enormous depth of the lake in many places 

 rendered it impossible to dredge, and whether with an efiicient 

 deep-water apparatus anything further could be obtained from 

 the abysmal recesses it is impossible to say. But it was very 

 soon apparent that the molluscan portion of the population 

 rapidly thinned out with increasing depth and distance from 

 the coast, and that beyond 100 feet one could often dredge 

 for miles over rocks and sand and mud without securing a 

 single shell. 



In many places the lake was floored with compact drifted 

 masses of shells and shell-fragments, consisting chiefly of the 



