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 : — 
1. Limnaea natalensis Krauss. 
2. Isidora nyassana E. Sm. 
3. ,, succineoides E. Sm. 
4. Physopsis africana Krauss. 
5. Planorbis (sp. indt.). 
6. Ancylus (sp. indt.). 
7. Ampullaria gradata E. Sm. 
8. Lanistes olivaceus Sow. 
9. ,, ovum Ptrs. 
10. ,, ellipticus Marts. 
IX. ,, nyassanus H. Dohrn. 
12. Vivipara unicolor 01 . 
13. Bithynia humerosa Marts. 
14. ,, Stanleyi E. Sm. 
15. Melania tuberculata Mull. 
16. ,, simonsi E. Sm. 
17. ,, nodicincta Dohrn. 
18. Melania pergracilis Marts. 
19. ,, polymorpha E. Sm. 
20. ,, turritispira E. Sm. 
21. ,, arcuatula Marts. 
22. Unio mossambicensis Ptrs. 
23. ,, leideri Marts. 
24. ,, lechaptoisi Ancy. 
25. ,, borelli Ancy. 
26. ,, kirki Lea. 
27. ,, nyassaensis Lea. 
28. ,, hypsiprymnus Marts. 
29. Spatha kirki Ancy. 
30. ,, nyassaensis Lea. 
31. Mutela alata Lea. 
32. Corbicula radiata Phil. 
33. ,, astartina 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 
