THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 179 



From all this it will be seen that, unless we are to assume 

 that the Halolimnic group came into Tanganyika from the sea in 

 very ancient times indeed, and that they are far older than their 

 characters in any way appear to warrant/ we are without any 

 direct evidence from geology that the sea, or even an arm of 

 the sea, has ever been in the Tanganyika region of the interior. 

 So far as positive evidence goes, geology is absolutely silent 

 upon this subject, it offers no evidence of any sort; and the 

 theory of marine contamination — if it occurred, let us say, 

 during a later period than Jurassic times — is thus diametrically 

 opposed to a geological theory of the nature of the African 

 interior which is at present accepted by many competent 

 authorities. 



The only way in which the nature and origin of the Halolim- 

 nic group can be really satisfactorily determined is, therefore, 

 through a minute knowledge of the morphology of the in- 

 dividual members of the group themselves, and the best types 

 belonging to the Halolimnic group for this kind of work are 

 the Gastropods, because these organisms, unlike the lake 

 Medusse, can be more or less directly compared with all sorts 

 of analogous organisms, ancient and modern, fresh-water and 

 salt. If it can be shown from the study of their morphology 

 that the Halolimnic Gastropods in Tanganyika are really mor- 

 phologically most closely related to the fresh-water Gastropods 

 at present known, then the theory of the ancient fresh-water 

 origin of the Halolimnic group is probably true. If, on the 

 other hand, it turns out that the Halolimnic Gastropods are 

 really most closely related to typically marine genera, then 

 there will be little doubt that the Halolimnic group originated 

 in the lake through marine contamination, and geological 

 conceptions will have to make room for the fact of the interior 

 of Africa having been connected with the sea as best they can. 



In arriving at the conclusions contained in the preceding 

 Gregory I do not believe that tlie north of Nyassa lies in the main eastern 

 rift, but iu one which through Lake Rukwa is continuous with the western 

 Tanganyika series. See Gregory's 'Rift Valley,' p. 7; also my paper iu the 

 ' Journal of the Royal Geographical Society,' September, 1897. 



^ See foot-note on page 161. 



