420 Proceedings of Royal Society of Edinburgh. [sess. 
Outside the 9-metre (30-feet) contour we find no plants except 
certain species of Oscillatoria and bottom Diatoms; all higher 
vegetation is limited Avithin this contour, and the slight trans- 
parency of the water is probably the main factor in determining 
this distribution. The majority of the snails also are limited 
by this contour, only Valvata piscinalis extending a little beyond ; 
the pulmonary snails never cross this boundary, the abyssal 
Limncea known from the Lake of Geneva being entirely absent 
from our lakes. The same contour also marks the boundary of 
nearly all the insect larvm, only Sialis penetrating so far. 
The deep bottom of our lakes is chiefly inhabited by Pisidimn, 
the larvae of Chironomus and Tanypus, the Oligochsete 
Psammoryctes fossor, Ostracoda ( Limnicythere relicta and some 
species of Candona), a few Planaria (Plagiostoma lemani), etc. 
The Daphnidae and the very minute forms of animal life, such 
as Protozoa, have not been studied. On the whole, I think I may 
say that our abyssal fauna, though imperfectly known, is still 
undoubtedly very like the abyssal fauna of the Swiss lakes. 
B. The Scottish Lakes. 
In comparing the associations of fresh-water organisms in the 
Scottish lakes with those in the Danish lakes, we shall find in 
nearly every particular the greatest contrast. 
1. The Littoral Region . — With regard to this region we may, 
in the first place, point out that the belt of vegetation which 
nearly always surrounds our lakes is often entirely absent from 
the larger Scottish alpine lakes, due to the precipitous or stone- 
covered shores, devoid of deposits of sand or decaying vegetable 
matter : even river deltas and other sandy flats are often almost 
bare of vegetation, partly, I suppose, because of the powerful 
erosion of the waves, and partly because the sudden changes in 
the level of the lakes is destructive to the amphibial plants. In 
the smaller and shallower lakes, for instance Loch Oich, in 
which we find some higher vegetation along the shore, this 
vegetation is not arranged in those elegant zones so characteristic 
of the Danish lakes. 
As far as I am aware, the stones have never been found 
clothed with blue-green Algse ; but when I had the opportunity 
