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. Journ. Micro. Sci., Vol. 41. 



