212 FRESHWATER FAUNA 



this would be fatal were it not for the fact that at least 

 one genus of the Hydrozoa, Cordylophora, seems to be 

 able to survive somewhat rapid changes in the saltness 

 of the surrounding water, and is introduced into our 

 rivers by vessels sailing up from the sea. 



Mr. Moore very definitely maintains that the whole of 

 the peculiar forms of Tanganyika are to be regarded as a 

 relic fauna found dwelling in a relic lake; i.e., that lake 

 Tanganyika was at one time an arm of the sea, and that 

 its peculiar fauna were originally marine inhabitants of 

 that sea. On the supposed similarity of some of the 

 Mollusca to Jurassic fossils, he assigns a date to the sea, 

 and regards it as having been in continuous existence 

 from an early part of the Jurassic period. Mr. Huddle- 

 ston, our leading authority on Jurassic shells, is unable to 

 find any very close resemblance between the peculiar 

 Tanganyika shells and the Jurassic fossils which have 

 been compared with them ; * on the other hand, some of 

 these shells already existed as freshwater forms at the close 

 of the upper Cretaceous period. Thus Paramelania has 

 been identified by White and Tausch with Pyrgulifera 

 (Fig. 78), which, we have seen, is found in approximately 

 contemporaneous beds in North America and Europe ; 

 the same is true of Syrnolopsis, already identified with 

 Fascinella ; further, the association of Spatha with these 

 forms in Europe naturally suggests a connection with 

 Africa. It is quite within the bounds of possibility, 

 therefore, that the Tanganyika fauna, or at least a part of 

 it, is a survival of the once widely-distributed freshwater 

 fauna of the uppermost Cretaceous deposits. The intro- 

 duction of this fauna into lake Tanganyika may have 

 been a comparatively late event. If much stress is to be 

 laid on the resemblance of Spekia to Lamellaria, then 

 even the Pliocene is suggested, and the presence of the 



* W. H. Huddles-ton, Geol. Mag. Dec., VI., vol. i. p. 338, 1904. 



