168 



J. E. S. MOORE. 



Nyassa Viviparas. No sponges grew upon these shells, and 

 beyond an occasional Melania such stretches of the lake were 

 without life of any sort or kind. The curious diminution of 

 the molluscs of Nyassa beyond the immediate coast-line is a 

 striking feature of the lake throughout, and a tolerably correct 

 idea of the bathymetric distribution of the individual genera 

 will be gathered from the accompanying table, which contains 

 an epitome of the observations made. 



Table II. — Bathymetric Distribution of Molluscs 



in Nyassa. 



100 200 300 400 500 600 700 800 900 1000 

 The numbers at the foot indicate the approximate limits of depth in feet at 

 which the molluscan genera are found, and the thickness of the lines 

 shows in what depth of water they most abundantly occur. 



The lines representing the bathymetric extent of each genus 

 are thickened so as to show where it is found in the greatest 

 abundance, and approximately at what depth it ceases to exist. 



From this table it will be seen that the purely fresh-water 

 molluscan population of Nyassa is more or less completely re- 

 stricted to a littoral band along the shores, and that the great 

 majority of the genera do not extend in depth beyond 200 

 feet. 



Thus Lake Nyassa, so far as molluscs go, is thinly peopled, 

 and great extents of its shore and deep bottom are altogether 

 uninhabited. The above facts of distribution, indeed, give the 



