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1927] Pulsbry-Bequaert, The Aquatic Mollusks ‘of the Belgian Congo 083 
Corbicula, aud Unio, all ordinary fresh-water types. The amount of 
material at hand, however, is so small that one cannot say to what extent 
the thalassoid shells may be represented in the same locality.’ 
As Tanganyika extends from north to south over nearly six degrees 
of latitudé, one might suppose that conditions of climate and salinity 
show enough differences at various points to produce appreciable changes 
in the fauna. That such is the case has been claimed by J. E. 8S. 
Moore: ‘In Tanganyika,” he says, “there are at least three very well 
marked varieties of the ally of the genus Virpara, Neothauma. One of 
these with the type of shell represented on page 261 (a) occurs exclusively 
at the south end of the lake, swarming in the broad and more or less shel- 
tered reaches into which the southern end of Tanganyika expands. In 
the narrow, surf-swept, and turbulent portion of the lake, which stretches 
between the north of Cameron Bay and Tembwi Neothauma is only 
found in the little bays and sheltered places occurring along both shores, 
and here the character of the form changes, the double-keeled shell of the 
former variety being replaced by the elongated type shown on page 261 
(b). Northward the lake terminates again in more or less sheltered ex- 
panses, like the Gulf of Ubuari, the deep bays near Ujiji, and the extreme 
northern extremity of Tanganyika. In these the form of the genus again 
changes, the two more southern varieties being replaced by the curious 
rounded form represented on page 261(c).’2 Moore also states that 
Tiphobia and Paramelania show similar geographical varieties. A closer 
examination of the available records gives no definite support to this view. 
Like most fresh-water mollusks, the shells of Tanganyika appear to be 
subject to considerable variation and this is true of the thalassoid as well 
as for the more usual fluviatile types. But it is by no means apparent 
that any of the forms thus produced within the same species are restricted 
to certain areas of the lake. In the case of Neothauma, E. A. Smith has 
called attention to the occurrence together, at the southern end of the 
lake, of both the keeled and unkeeled varieties, and this is also brought 
out by the extensive collections made by Stappers and by Charles Hedley’s 
collections from Albertville. | 3 
Tanganyika is, after Lake Baikal, the deepest fresh-water lake of the 
earth. About midway its length, a transverse crest, at a depth of 136 m. 
to 600 m., divides it into a northern and a southern basin. The northern 
1K. A. Smith, 1906, Proc. Zool. Soc. London, I, p. 185. ; 
J. E. 8S. Moore (1903, ‘The Tanganyika problem,’ p. 8) also mentions having met at several points 
along the shore, between Ujiji and Usumbura, modern lake-beds which are ‘‘now 100 feet above the 
water-line, and contain the same peculiar halolimnic shells which are now found inhabiting the lake. 
21903, ‘The Tanganyika Problem,’ pp. 149-150. 
31906, Proc. Zool. Soe. London, I, pp. 180 and 184. 
