1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 093 
as ancestral to or derived from such Tanganyikan types as Chytra, Tipho- 
bia, Spekia, ete.4 | 
The perplexing questions raised by the origin and evolution of 
Tanganyikan mollusks are closely linked up with the more general 
problem of the origin of the lake itself. We do not intend to enter deeply 
into it, since it has been often discussed at length, more recently again by 
L. Germain. Although the marine-like appearance of some of the 
mollusks had been noticed several times before, it was the discovery of 
meduse that prompted R. T. Giinther? to argue that Tanganyika was a 
“residual sea,’”’ the remnant of an extension from the Atlantic, which at 
one time, he claimed, covered most of the present Congo basin. Left 
behind by a subsequent regression of the sea, the lake, he said, gradually 
lost most of its former salinity through dilution with rain water and 
drainage to the ocean, while the fauna retained many of its marine char- 
acteristics and acquired through migration’a number of strictly fluviatile 
types. We know, however, that the meduse, though undoubtedly of 
marine origin as a group, are much more widely distributed in fresh waters 
—especially in Africa—than was at first suspected. These animals 
can no longer be used as an important argument in favor of Giinther’s 
theory, since there has been plenty of opportunity for their migration 
along fresh-water streams into Tanganyika. The theory was considerably 
stressed by J. E.S. Moore,’ who held the opinion that Tanganyika repre- 
sented an ancient extension of the Jurassic ocean. The thalassoid mol- 
lusks he regarded as “‘halolimnic”’ survivors of Jurassic, marine forms. 
Moore has even attempted to show that “‘the very remarkable facies 
which the shells of the halolimnic gastropods possess is unmistakably 
again presented by a similar number of gastropods which are character- 
istic of the deposits left by the old Jurassic seas.’’? His efforts in that 
direction appear somewhat futile, since in external appearance the thalas- 
soid shells could equally well be matched with fossils of other geologic 
periods or with present-day marine forms. It is sufficient to mention 
1We cannot well agree with Germain’s statement that Pseudogibbula is “‘un véritable Troque d’eau 
douce”’ analogous to Chytra and Limnotrochus. It appears to be either a littorinid (as Dautzenberg 
thought), which like Cremnoconchus has becomeadapted to fresh water and is most likely arecent deriva- 
tive from a marine ancestor; or possibly it may belong to the Amnicolide. It remains without meaning 
until the dentition can be examined. uae 
2Giinther, R. T. 1894. ‘Further contribution to the anatomy of Limnocnida tanganyice.’ Quart. 
Journ. Micr. Sci., N.8., XXXVI, pp. 271-293, Pls. xvi1—x1x (see pp. 289-290). 
3Moore, J. E. 8. 1898 (March). ‘On the zoological evidence for the connection of Lake Tangan- 
yika with the sea.’ Proc. Roy. Soc. London, LXII, pp. 451-458. : 
1898 (June). ‘On the hypothesis that Lake Tanganyika represents an old Jurassic sea.’ Quart. 
Journ. Micr. Sci., N. 8., XLI, pp. 303-321, Pl. xxi. 
1908. ‘The Tanganyika Problem.’ (London), xxiii+371 pp.; Pls. and maps. ; : 
4A somewhat similar theory was advanced by H. Nicolas (1898, Origine marine de certaines espéces 
de mollusques, en cours de transformation, du lac Tanganyika.’ C. R. Ass. Franc. Av. Sci., 2° p., pp. 
508-525). Pelseneer |1906, Bull. Ac. Belgique, Cl. Sci., (1905), pp. 706-707], although condemning 
Moore’s hypothesis, yet believes that Tanganyika was partly populated by forms of marine origin that 
migrated from the west. 
