OP THE HALOLIMNIC FAUNA OP LAKE TANGANYIKA. 345' 



be seen in the texture of the shells, the colouring, the condition 

 of the epidermis, etc., which may be noted in some of, but not 

 in all, the gc^nera. 



The strongest argument of all in favour of an exceptional 

 original is the fact that, so far as is known at present, the- 

 halolimuic gasteropods are confined entirely to Lake Tauganyika,. 

 and this circumstance will incline us to look to the Congo basin, 

 as being the place where the mystery may some day be solved.* 



Before attempting to grapple with this part of the subject 

 which will involve the study of the geological structure of large 

 portions of Equatorial Africa, there are two independent con- 

 siderations on which I might say a word. 



Distribution of Jurassic faunas in intermediate areas. — The 

 first of these considerations relates to the distribution of knoion 

 Jurassic faunas in areas intermediate between the Anglo- 

 Norman basin and Lake Tanganyika, so far as such an 

 investigation can be made, and thus endeavour to ascertain 

 if this will throw any light upon the possible Jurassic origin 

 of the halolimuic gasteropods themselves. From the quairiea 

 of Dorset to the depths of Tanganyika is a far cry and there 

 should be some half-way houses, some stepping stones, as it 

 were, to bridge over the vast distance that lies between them. 

 Mere zoological conjecture, as I have already pointed out, is 

 not sufficient. We must have some palreontological evidence in 

 corroboration of the intimate relationship claimed to exist 

 between the two gasteropod faunas, i.e., between the real fossils 

 and those molluscs which are only archaic in their internal 

 development. In the first place, then, I may say that in this 

 country the peculiar gasteropod fauna which characterises the 

 Inferior Oolite of the Anglo-Norman basin can hardly be 

 traced above the Lower Oolites, though a stray form may linger 

 in the Callovian or even the Corallian of Yorkshire. In 

 middle France a repetition of this peculiar fauna is seen in the 

 Callovian of Montreuil-Bellay. When we trace the Jurassic 

 faunas into the south-west of France, although there is much 

 in common with parts of the Inferior Oolite of our own 

 country, yet the analogy, as far as gasteropods are concerned, 

 is mainly confined to such genera as Nerincea. 



* The fact that a species of jelly-fish identical with the one in 

 Tanganyika has recently been discovered in the Victoria Nyanza, but 

 slightly affects the argument as regards the halolimnic gasteropods. "We 

 can scarcely doubt that the more mobile organisms have had opportuni- 

 ties of establishing themselves from the great centres of distribution in 

 a way which is denied to the more sedentary molluscs. 



