* 5 ° 
THE TANGANYIKA T ROB LEM. 
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 expanses, 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 
generally replaced by the curious rounded form represented 
on page 261 ( c ). The same phenomenon is again remarked 
in the distribution of the beautiful Typhobia shells of Tangan- 
yika. In the north we have a type which differs from that 
in the south. So again with regard to the distribution of 
the Tanganyika genus Paramelania and its allies ; the 
shell of Paramelania damoni is represented on page 242, 
and occurs more or less abundantly throughout the lake ; 
but in certain areas it is to a greater or less degree replaced 
by several definite varieties ; one of these, represented on 
page 238, is so peculiar and well marked as to constitute 
a new generic type. This form, Bythoceras , occurs only 
in the southern half of the lake, and is practically exclu- 
sively related to Cameron Gulf and the bays which lie 
opposite the gulf on the east coast. Another variety which 
is much more closely related to Paramelania damoni is 
the form Paramelania crassigranulata , and this variety 
is also restricted to certain areas on Tanganyika, being 
encountered about Karema on the east coast of the lake. 
From these observations it would appear to be indicated 
that the cause of the production of a greater number 
