1897.] OF THE TANGANYIKA EXPEDITION. 4:37 



the physiographical characters of the lakes themselves. We have, 

 in the interior of the continent, a number of sheets of water at 

 ■very different altitudes, displaying considerable variations in the 

 climatic and other conditions which affect the faunas they contain. 

 Some of them are salt, some brackish, and some fresh. Many of 

 them are connected with each other by rivers, which flow from one 

 to the other and find their way out of the lowest into some great 

 channel towards the sea. The basins in which these lakes are con- 

 tained are, moreover, readily divisible into two distinct kinds, some 

 being broad and shallow, while others are long, narrow, and deep. 

 The former appear to be filled by collections of rain-water in the 

 depressions of an elevated plateau ; but the latter have certainly been 

 formed in quite a different way, their origin having attracted the 

 attention of geologists for some time. It appears that the forma- 

 tion of these valleys is to be associated with a series of geological 

 commotions which have affected an immense area of the African 

 continent, their action having extended from Nyasa to the Eed 

 Sea, and even farther north. It is, moreover, not probable that 

 all these depressions were formed simultaneously, and it is quite 

 likely that some of them are as old as the sedimentary deposits of 

 the Jurassic seas. They exist now as a series of long narrow 

 valleys running north and south along the whole continent, and 

 some of them are so deep that their bottoms are, like that of 

 Nyasa, many hundreds of feet below the level of the sea. Such 

 lakes, owing to their great depth, are not likely, as the superficial 

 waters are, to be subject to much change; and we should therefore 

 expect to find a greater abundance in the faunas they possess, and 

 this in fact is found to be actually the case. There is, however, 

 nothing in their surroundiugs, or in the climatic conditions by 

 which they are affected, which could widely differentiate the faunas 

 of the great deep lakes from those of the equally great shallow 

 ones. There is nothing in the physiographical features of either 

 class of lakes to suggest that the animals which inhabit them will 

 exhibit any greater specific differences than those which would 

 naturally be expected in sheets of water at different altitudes in 

 different climates, and spread over an immense area of land. 



Next to Tanganyika, restricting our attention at present to the 

 Molluscs, we find that the fauna of Lake Nyasa is the most com- 

 pletely known. It is obvious from the first that we have, in this 

 lake at least, nothing but what are in the strictest sense lacustrine 

 forms. Turning to the other lakes, we find in Shirwa represen- 

 tatives of PalucUnce similar to those in Lake Nyasa, together with a 

 small PlanorUs near tbe shore. In the little Lake Keler, near the 

 south end of Tanganyika, there are species of Planorhis, but the 

 fauna of Lake Eukwa is unfortunately unknown. In Lake Bau- 

 gweolo it has recently been reported that there are no shelled 

 molluscs of any sort ; but in Lake Mwero, which is not very far to 

 the north, there are several examples of Nyasan genera of this 

 group. The fauna of the small lakes north of Tanganyika is not 

 known, but in the Albert Edward Nyanza there are a large number 



