FAUNA OF THE AFRICAN LAKES. 



549 



family, they belong mainly to that of the Tiariclse. Tanganyika 

 is the only lake of those under review which contains represen- 

 tatives of all the genera enumerated in the table of distribution. 

 — Victoria Nyanza, with a far smaller number of individual forms 

 (28), can nevertheless show quite a comprehensive series of the 

 normal fresh- water types, each of the genera being represented. 

 The thalassoid genera and species are conspicuously absent. — 

 Nyasa, with only slightly lower figures (24), does not difler very 

 markedly from Lake Victoria. The genera Cleopatra smd Ancylus 

 are unrepresented, but the lake is lich in species of Lanistes and 

 Tiara, especially the latter, of which 9 forms (7 endemic) are 

 known. — Albert Nyanza, and Edward ISTyanza exhibit a progi'essive 

 reduction in the number of species which they contain, but 

 otherwise possess no outstanding features. — Lake Kivu, finally, is 

 the extreme case, with a very reduced Gasteropod fauna. Of the 

 two species which are known from its waters, Tiara tube/cidata 

 is the more widely distributed, it being in fact the only form 

 enumerated which has been found outside the continent of Africa. 



It was in the first degree the Gasteropoda, which constituted 

 Moore's argument for a halolimnic {i.e. relict) fauna living in 

 Tanganyika side by side with types which are common in tropica^l 

 fresh- waters. It is thus necessary to consider whether the balance 

 of evidence is still in favour of the view^s regarding these Mollusca 

 which Moore originally expressed. He held that the peculiar 

 forms in question were essentially primitive types, and after com- 

 parisons based on their anatomy he stated : — *' It is difficult, or, 

 I may say, impossible, to view these extraordinary molluscs as 

 either the forerunners or the derivatives of the fresh-water 

 molluscs which we find in the lakes and rivers all over the world 

 to-day. They are, however, readily intelligible if we regard them 

 as the forerunners of several marine groups, such as the Strom- 

 bidfe, the Naticas, and the early Ciriths, to which I have referred " 

 (135, p. 466). 



These statements did not go by any means unchallenged, nor 

 did the astonishing suggestion of a resemblance between some 

 species and certain Jurassic fossil shells. Edgar Smith, in his 

 important paper on the Mollusca of Lake Tanganyika, pointed 

 out that Moore's conclusions Avere drawn from a study of only 9 

 out of 23 so-called halolimnic genera, adding that it became 

 "mere conjecture" to suppose a relationship wdtli marine forms 

 on account of the appearance of the shell (170, p. 78). As 

 an expert conchologist, Smith dealt unfavourably too with the 

 supposed resemblaiices to Jurassic fossils. The matter was 

 approached from the geologist's standpoint by Hudleston, who 

 had made a speciality of Jurassic Mollusca (102). In a couj- 

 prehensive paper on the origin of the halolimnic fauna of 

 Tanganyika, he failed to find evidence of a satisfactory character 

 in favour of Moore's views. In an appendix, Hudleston also 

 dealt seriatim with the comparisons between living and Jurassic 

 Gasteropods, only to reject likewise Moore's conclusions. In a 



