PREFACE. 
vii 
“ Eurycolpic folds,” a term recently suggested to me, and which 
I have intentionally introduced into the present work in preference 
to either “ graben,” or “ rift valleys,” both of which are obviously 
misnomers, since the valleys in question are generally produced 
by folding, due to lateral pressure, and not either through rifting, 
or vertical collapse. 
Since our return, and in connection with both the first and second 
Tanganyika expeditions, the large number of new fishes obtained 
have been elaborately worked out by Mr. Boulenger of the British 
Museum. The descriptions contained in the present volume 
being his simply, while the more conspicuous genera and species 
have been accurately illustrated by Mr. Green. The Crustacea 
were examined by Messrs. Cunnington and Caiman, the Sponges 
by Mr. Evans, Professor Minchin and Herr Weltner in Berlin. 
For the description of the Mollusca, the Polyzoa and Protozoa, 
I myself have been responsible, but I have received unlimited 
help from Professor Ray Lankester himself, Mr. Edgar Smith 
and others at the British Museum. The anatomy of the cele- 
brated jelly-fish was examined some years ago by Mr. Gunther 
in Oxford, and in what appears in the present volume, I have 
consequently been able to draw largely upon his original descrip- 
tion, only adding some drawings of the living Medusa together 
with information about its life history and development, which I 
acquired on the spot. We have been further indebted to Mr. 
Prior for the complete identification of the rock specimens collected 
by Mr. Fergusson ; and, as 1 have explained in the text, we have 
been most materially helped by Mr. Huddleston in the comparison 
of the Tanganyika gastropods with the shell remains of the Jurassic 
seas. Last, but not least, I have to thank Professor G. B. Howes, 
who after my return from both my first and second Tanganyika 
expeditions, allowed me the unlimited use of the Huxley laboratory 
in the Royal College of Science and of his own experience and 
advice. 
The title which I have chosen for the present work, “ The Tan- 
ganyika Problem,” expresses the fact that there is a puzzle, a 
mystery, attached to Tanganyika, and the elucidation of this 
mystery has formed the single motive for several independent 
lines of enquiry described in the present volume. The Tan- 
