172 J. B. S. MOORE. 



geographical isolation, but by their bathymetric distribution 

 also ; the conclusions to which the facts of their geographical 

 distribution seem to point being thus completely substantiated 

 from another point of view. 



There are, however^ yet other ways in which the fact that 

 the Halolimnic fauna is entirely distinct from, and unconnected 

 with the more normal series becomes clear. For in many 

 branches of biological inquiry we are often rightly guided 

 by impressions which, like the types of human physiognomy, 

 are real enough, but quite incapable of definite expres- 

 sion. Impressions of this character are at once produced on 

 reaching Tanganyika, as I did, after studying the fauna of 

 several neighbouring lakes. For there is a singular and oceanic 

 profusion of life in Tanganyika, which is quite peculiar, and it 

 quickly becomes evident that this numerical increase in the 

 aquatic population does not affect the normal fresh- water stock, 

 it is solely produced by the astonishing abundance of the 

 members of the Halolimnic group. In contrast with the shallows 

 of Nyassa, the creeks and bays of Tanganyika swarm with crabs 

 and prawns, and the open sandy beaches are strewn with 

 empty Halolimnic shells ; dead detached fragments of the deep- 

 water sponges are tossed up by hundreds on the shore. And 

 on the extensive rocky coasts the barely submerged stones are 

 covered with the so-called Lithoglyphus and Nasopsis, just 

 as the half-tide rocks swarm with Natica and Litorina on 

 an English beach. Further, on putting out into the lake 

 itself, the deep open water is filled and discoloured with clouds 

 of pelagic Protozoa (chiefly Peridinia and Condylostoma); 

 and during the dry season swarms of the lake jelly-fish are 

 seen pulsating at all depths. 



Uecapitulating, it may be said, then, that the facts of 

 the geographical and bathymetric distribution of the great 

 lake molluscs lead to the following results : — That among all 

 the fresh-water lakes of the African continent which have 

 hitherto been explored there exists a type of fauna which is 

 curiously similar throughout. It differs only in the specific 

 representation of the same genera which these lakes contain. 



