THE FRESH- WATER LOCHS OF SCOTLAND. 139 



of Tomnadashan 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 Diaptomus gracilis, Cyclops 

 strenuus, Daplmia lacustris, Bosmina obtusirostris, the Rotifers 

 ('uuochilus (two species), Anurcea cochleare, and Notholca longispina, 

 and the Diatom Asterionella gracillima. These are found at all seasons. 

 In the summer, Holopedium, Leptodora, Bythotrephes, and Polyphemus 

 are as generally distributed. 



Only less common are Asplanchna priodonta, Polyarthra platyptera, 

 Pendinium tdbulatum, Ceratium Jiirundinella, Mallomonas. Some 

 Desmids, mostly of the genus Staurastrum, but including also species of 

 Mirrasterias, 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 



