ABSTRACT
The present paper forms the first instalment of the zoological report of an expedition which, through the generous support of the Royal Society and the British Association, I was able to make to Lake Tanganyika during 1895 and 1896.
It is quite possible that many of the old so-called fresh-water deposits are in reality marine, since the forms which became exclusively fresh water as time went On probably made their appearance in the sea first, as so many of the more recently derived fresh-water types have done,—prawns, for example.
The affinities of the deep-water sponge which I obtained in Tanganyika have not yet been determined, but its striking external character and its remarkable deep-water habitat have inclined me to regard it as a member of the anomalous section of the fauna which the lake presents.
‘ Ann. des Sci. Nat.,’ septième série, ix, x, 126, 1890.
On further examination it appears—(1) That Hie genus Paramelania of Tanganyika is similar to the cretaceous Pyrgulifera; (2) but that the genus Pyrgulifera, so far as some of its representatives go, is oonchologically indistinguishable from the old marine Jurassic genus Purpurina, and that the Nanopsis of Tanganyika corresponds to one section of this genus, the Paramelania to the other. (See Hudleston’s figs., Plates I and ii, and text p. 85—95, ‘ Jurassic Gasteropoda,’ Palseontographical Society, vol. xli, 1887.) It would thus appear that the marine genus Purpurina became a fresh-water form, as so often happens in Cretaceous times. We find, however, that other Halolimnic Gasteropods, Bathanalia, the socalled Lithoglyphus, and Limnotrochus, are also indistinguishable from marine Jurassic forms, which are not found in any Cretaceous formation, fresh-water or marine. Consequently the geological evidence on this matter distinctly favours the old marine origin of the Halolimnic fauna; but it places their original marine existence much further back than I had even dared to suggest. I shall discuss this most interesting line of investigation fully in a special memoir.
I am quite aware that this statement cuts at the roots of many geological determinations ; but I am prepared to maintain that the criticism is sound.
Murchison, President’s Address, ‘ Journ. Royal Geog. Soc.,’ vol. xxii, 1852.
‘ Jouru. Royal Geog. Soc.,’ vol. xxxix, 1864, pl. xxxvii, pp. 201—205.
Drummond, ‘ Tropical Africa.’
It appears, however, that these fossils have been by no means satisfactorily described.
See Suesa,’ Die Brücke des Oust Afrika.’
The Great Rift Valley.
The southern two thirds of Nyassa is not in a rift; and in contrast to Gregory I do not believe that the north of Nyassa lies in the main eastern rift, but in one which through Lake Rukwa is continuous with the western Tanganyika series. See Gregory’s ‘ Rift Valley,’ p.’ 7 ; also my paper in the * Journal of the Royal Geographical Society,’ September, 1897.
See foot-note on page 161.
