THE FRESH-WATER LOCHS OF SCOTLAND. 
139 
of Tomnadastian and Beinn Bhreac. The soundings show that the 
deepest part of the basin, which is bounded by the 400-feet contour- 
line, lies along the course of this fault. Under these circumstances. 
Loch Tay cannot be regarded as a typical example of a rock basin. 
The other rock basins, however, seem to us to furnish strong 
evidence in support of the theory that they have been eroded by ice- 
action. 
Biology of the Lochs of the Tay Basin. 
By James Murray. 
While it was not compatible with the bathymetrical work of the 
Lake Survey to study in detail the biology of the lochs, it has been 
customary to make collections of the plankton of each loch, a coarse and 
a fine net being used in each case. It is thus possible to compare only 
the biology of the open water of the different lochs. The number of 
species living in the open water is not very great, and does not vary 
in different lochs so much as might have been expected. The fauna of 
the shallower lochs is usually much richer than that of the deeper ones, 
owing to the occurrence in them of many species which in larger lochs 
would be confined to the shore region. Even thus limited, it is found 
that the lochs differ sufficiently from one another to render a com- 
parative review of them of much interest. Each loch has a distinct 
character, which, notwithstanding a considerable amount of seasonal 
variation, is pretty constant. 
A small number of animals and plants occur so constantly in the 
open water of all our lakes, large or small, that they mainly determine 
the character of the plankton of this pelagic region. They are so 
generally present that the absence of any one of them is occasion for 
remark. The most important of them are — Diciytomus gracilis, Cyclops 
strewuus, Daphnia lacustris, Bosmina ohtusirostris , the Rotifers 
Conochilus (two species), Anurcea cochleare, and N otholca longispina, 
and the Diatom Aster ionella gracillima. These are found at all seasons. 
In the summer, Holopedium, Leptodora, Bytliotrephes, and P olyphe^nus 
are as generally distributed. 
Only less common are Asplanchna priodonta, P olyarthra platyptera, 
Peridinium tahulatum, Ceratium hirundinella, Alallomonas. Some 
Desmids, mostly of the genus Staurastrum, but including also species of 
Micrasterias, Xanthidium, and Closterium, are generally present. The 
Rotifers Floscularia pelagica and Notops pygmceus are of frequent 
occurrence. Although all of those species may be present in most of 
the lochs, the varying proportions in which they occur in the plankton 
give rise to great differences of character in the lochs. Other species of 
