2 2o 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 were 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, when 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 Typhobia, Limnotrochus, Neothauma, Spekia, 



