422 Proceedings of Royal Society of Edinburgh. 
two species of Ephemeridse and Phryganidse. In the small bays 
of the lakes, where the bottom may he seen well covered with 
vegetation, for example Littorella , Myriophyllum , etc., we often 
find a comparatively rich fauna of insect larvae, Cladocera, and 
Rotifera; in such localities the fauna in these respects does not 
seem to he much inferior to that found in the Danish lakes. 
Between the littoral fauna of the Highland lakes as compared 
with that of the Danish lakes, the main difference appears to he in 
the Mollusca, which play a very prominent part in our lakes, hut 
are extremely rare in the Highland lakes. Along the shores of 
Loch Ness and the other lochs of the Caledonian Canal I never 
found a single mollusc shell, and on exploring the shores only a 
few living specimens of Limncea ovata and Planorbis contortus 
were to he found. Still, I expect that a closer examination hy a 
malacologist would reveal more species, and that in the shallow 
water, in depths of 15 or 20 feet, species of Vcdvata , Bithynia , 
etc. would he found, hut all the larger species of Planorbis and 
Limmea seem to he entirely wanting. At any rate the molluscan 
life in the Highland lakes generally is so extremely poor that it 
cannot possibly influence the general conditions of life in the zone 
in which it is principally found. 
This special difference between the Scottish and Danish lakes I 
consider to he due to the large amount of humic acid in the water 
of the Scottish lakes, to the total absence of lime in the water and 
on the floor of these lakes, to the absence of all lime-secreting 
Algae and of lime-encrusted blue-green Algae covering the stones, of 
Characea, etc., on which the snails in our lakes principally feed, 
and to the, generally speaking, extremely poor vegetation. That 
the first-mentioned is the principal cause is evident from the fact 
that even in lakes rich in vegetation the molluscan life is greatly 
inferior to that in the Danish lakes. 
2. The Pelagic Region . — The investigations of Mr James 
Murray,* assistant on the Lake Survey staff, of Messrs West, and 
my own cursory examinations, have shown that there is a great 
resemblance, and at the same time a great difference, between the 
plankton of the Scottish and of the Danish lakes. Nearly all the 
common plankton-organisms of the Scottish lakes also occur in the 
* I desire to express my thanks to Mr Murray for information supplied to me. 
