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Show 1897.] OF THE TANGANYIKA EXPEDITION. 437 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 contained 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 formation of these valleys is to be associated with a series of geological commotions wdiich have affected an immense area of the African continent, their action having extended from Nyasa to the Red 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 w e 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 surroundings, or in the climatic conditions by which they are affected, wdiich 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, w e find that the fauna of Lake Nyasa is the most completely known. It is obvious from the first that w e have, in this lake at least, nothing but what are in the strictest sense lacustrine forms. Turning to the other lakes, w e find in Shirwa representatives of Pcdudince similar to those in Lake Nyasa, together with a small Planorbls near the shore. In the little Lake Keler, near the south end of Tanganyika, there are species of Planorbls, but the fauna of Lake R u k w a is unfortunately unknown. In Lake Ban-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 |
