OF THE HALOLIMNIC FAUNA OP LAKE TANGANYIKA. 343 



The arguvient limited to the halolimnic gasteropods. — Although, 

 therefore, the subsidiary fauna of exceptional character may 

 help to strengthen the argument in favour of the marine origin 

 of the entire halolimnic group, yet the most important link in 

 this chain of evidence is to be sought in the hcdolimnic gasteropods, 

 which are considered so greatly to resemble Inferior Oolite 

 forms, and which on the strength of this resemblance are held 

 to be derived from a well known gasteropod fauna of Jurassic 

 age. The malacological evidence, as regards the Tanganyika 

 species, has been well worked out by Mr. Moore, and the 

 conclusions as to the peculiar mixed and to a certain extent 

 archaic structure of their anatomy must undoubtedly have 

 great weight. But at this point the argument fails us, for when 

 we are disposed to institute a comparison between living and 

 fossil species we must in the main fall back on conchology alone. 

 One point of importance must be noted here, viz., that, since 

 the connection between the halolimnic fauna of Tanganyika and 

 the old Jurassic marine fauna is coniined to univalves, one 

 might almost have expected that some lamelUbranchs, and 

 particularly Trigonia, if only in a modified form, might have 

 accompanied their moUuscan relatives. For it can hardly be 

 contended that Trigonia would suffer more from translation to 

 fresh-water conditions than the numerous species of gasteropods 

 which are correlated with Jurassic forms. Moreover, if conchology 

 is to be our guide in this matter, it is to be regretted that the 

 author of the " Tanganyika Problem " should have endeavoured 

 to minimize the value of a branch of science on which his 

 conclusions with reference to the Jurassic origin of these 

 Tanganyika shells must in the main be based.* 



The above considerations apart, it must be admitted that 

 there are some genera of Tanganyika gasteropods which have 

 a striking external resemblance of form and ornamentation to 

 certain well-known genera which more especially characterize 

 the Inferior Oolite of the Anglo-jSTorman basin ; and if such 

 resemblance is not fortuitous there seems a fair reason for 

 regarding them as the possible descendants of such genera or 

 their allies. Consequently, some portions of Mr. Moore's 

 latest work are devoted to a detailed comparison between the 

 Tanganyika shells and their presumed Jurassic analogues. The 

 text is accompanied by excellent illustrations, the shell and 

 the fossil being drawn side by side. As a detailed criticism 

 of these comparisons might be somewhat tedious to the members 



* Geographical Journal for 1903, p. 682 et seq. 



