220 
THE TANGANYIKA PROBLEM. 
conclusion at which Joseph Thomson had arrived con- 
cerning the past history of the Central African region 
round Tanganyika during his famous journeys, and 
suggested the possibility that when more was known 
about the animals which these new shells contained, we 
might have to regard the region as the site of some former 
extension of the sea. 
The gastropod shells peculiar to Tanganyika, which 
were described by Smith, from the collections of Coode 
Hore, Thomson, and including those already described 
by Woodward, were represented by the following generic 
forms: — Ty phobia, Limnotrochus, Spekia, Stanley a, Tan- 
ganyicia , Nassopsis, Neothauma and Melania admira- 
bilis. Through the collections made by the French mis- 
sionaries and others, further varieties of these forms were 
subsequently added, and from the specimens which gradually 
accumulated in the Paris Museum, the conchologist, Bour- 
guignat, described a very large number of new species and 
genera, but the characters which w’ere used by this author 
as sufficient to define species and genera have not generally 
been held to be valid, even in a conchological sense ; they 
throw’ no light on the matter in hand, and it is not necessary 
to discuss them further here. 
In 1895, wTen the first Tanganyika expedition started, 
our knowledge of the molluscan fauna of the lake had not 
advanced, we knew nothing of the anatomical characters of 
any of the animals belonging to the unique shells which the 
lake had been shown to contain, and consequently the purely 
conchological determination of their affinities was, as Smith 
frankly implied, to be considered, to a great extent, pro- 
visional. During the first Tanganyika expedition I 
obtained material for the complete anatomical study of the 
genera Ty phobia, Limnotrochus, Neothauma, Spekia, 
