126 
THE TANGANYIKA PROBLEM. 
the open beaches and wears out the hard masses of the 
rocky coast into fantastic points and bays. An examina- 
tion of the 800 to 900 miles of coast line which 
Nyassa possesses showed that the fauna, including the 
crocodiles and the fishes, was almost exclusively restricted 
to a narrow littoral zone, its molluscan section living, 
in fact, in groups in bays and sheltered inlets, and 
being hardly ever found alive along the sandy surf-swept 
beaches which occur in many portions of the lakes. These 
molluscs, moreover, did not extend into the deep water of 
Nyassa, and it was found, both on the first and second 
Tanganyika expeditions, that beyond 100 to 150 feet 
the lake was practically a fresh- water desert, there being 
encountered in its deeper water nothing but a few dead 
shells, the fragments of crabs’ shields and legs and other 
organic refuse, enclosed in fine, grey mud.* 
East and south of Nyassa there exists, at about the same 
level, Lake Shirwa. The faunistic characters of this lake 
have already been described in Chapter II., and conse- 
quently need not be fully repeated here, all that it is 
necessary to remember being the fact that the fauna of 
Shirwa unquestionably appears to have been at one time 
identical with that of Nyassa. This is shown, as we 
have seen, by the semi-fossilised remains which occur 
around the shores of Shirwa, but, owing to the geographical 
change which has separated Shirwa from Nyassa, and 
which has finally resulted in Shirwa having no outflow, 
this lake has become extremely salt, and the old Nyassan 
fauna has been killed out of it, except in the curious fresh- 
water oases which are still maintained at the mouths of the 
permanent rivers flowing into the lake. 
* For tables of the bathymetric distribution of mollusca in Nyassa see paper in 
Quart. Jour n. Micro. Sci., Vol. 41. 
