38 THE NAUTILUS. 



" Instead of the Halolimnic molluscs being restricted to the shallow 

 creeks and bays about the coast, they swarm on the rough surf- 

 swept rocks and on the open beach. And what is more remarkable 

 than this, they extend in great profusion to the deepest portions of 

 the lake. Thus, dredging in water which varied in depth from 800 

 to 1,200 feet, I always obtained plenty of Typhobia, Paramelania, 

 Bathanalia, and Bytkoceras among the Gastropods, as well as the 

 so-called Unio Burtoni among the Lamellibranchiata ; and how far 

 these genera extended beyond these depths I cannot say, but they 

 showed no signs of dying out, but rather the reverse. On the lake 

 floors which were not so deep as this, from 200 to 300 feet below the 

 surface, but which were yet deep enough to have yielded nothing by 

 dredging in Nyassa, there was an abundance of Limnotroclms, 

 Syrnolopsis and Neothauma, together with those varieties of Melania 

 which inhabit Tanganyika. It is thus rendered apparent by these 

 observations that the Halolimnic molluscs are all either surf swept 

 rock dwellers, or entirely deep-water forms. It is thus apparent 

 that the Halolimnic molluscs are completely dissociated from the 

 normal fresh-water forms, along with which they exist in Tanga- 

 nyika, not only by their singular geographical isolation, but by 

 their bathymetric distribution also ; the conclusions to which the 

 facts of their geographical distribution seem to point being thus com- 

 pletely substantiated from another point of view. There are, how- 

 ever, yet other ways in which the fact that the Halolimuic 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 ex- 

 pression. Impressions of this character are at once produced on 

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

 neighboring lakes. For there is a singular and oceanic profusion 

 of life in Tanganyika, which is quite peculiar, and it quickly be- 

 comes 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-wnter sponges are tossed up by hundreds on 



