THE TANGANYIKA PROBLEM. 



I2 5 



the fresh- water crabs. There are no shrimps or prawns in 

 the lake, and when we pass to the molluscan section of 

 the fauna we find it to be constituted by the following- 

 characteristic fresh-water genera and species : — 



Limnaea natalensis Krauss. 

 Isidora nyassana E. Sm. 



,, succineoides E. Sm. 

 Physopsis africana Krauss. 

 Planorbis (sp. indt.). 

 Ancylus (sp. indt.). 

 Ampullaria gradata E. Sm. 

 Lanistes olivaceus Sow. 



,, ovum Ptrs. 



,, ellipticus Marts. 



,, nyassanus H. Dohrn. 

 Vivipara unicolor Ol. 

 Bithynia humerosa Marts. 



,, Stanleyi E. Sm. 



Melania tuberculata Miill. 



,, simonsi E. Sm. 



,, nodicincta Dohrn. 



1 8. Melania pergracilis Marts. 



There are no worms in Nyassa, and I never came across 

 any polyzoa, nor did I, after careful searching, find any 

 hydroids, and there appeared to be only one very incon- 

 spicuous spongillid of doubtful affinity. Besides the above 

 forms, there were representatives of the usual fresh-water 

 protozoa, and nothing else. Nyassa, however, is extremely 

 interesting from a zoological point of view. The fauna, as 

 will have been seen, is typically that of a pond, and con- 

 sequently it is highly instructive to ascertain how these 

 fresh-water pond animals which stock it, and which usually 

 live in the still waters of pools and puddles, have succeeded 

 in colonising the deep waters of the huge lake, which sink 

 in places to more than 400 fathoms, and over which a 

 heavy surf is often running at all seasons of the year, a 

 surf, in fact, which breaks in ocean-like lines of foam on 



