38 THE NAUTILUS. 
“ Tnstead of the Halolimnic molluses being restricted to the shallow 
ereeks 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 Bythoceras 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 Limnotrochus, 
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 molluses 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 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 
physioguomy, 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 Halolimnie group. 
“Tn 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 
