THE FRESH-WATER LOCHS OF SCOTLAND. 331 



western slope of Beinu Laoghal, a second passing through the hollow 

 occupied by the loch of that name, and a third round the eastern slope 

 of Benin's Tomaine. The stages in the gradual retreat of the mass of 

 ice that moved down the valley of Loch Laoghal are clearly marked by 

 a succession of moraine terraces, which enclose the small lochans shown 

 on the chart to the east of Loch Creagach and Loch Slaim. 



Loch an Dithreibh is a rock basin lying in hornblendic and micaceous 

 gneisses, whose strike is nearly north and south and nearly parallel to 

 the direction of the lake. They are admirably exposed on the great crag 

 on the east side of the loch. The solid rock is not exposed at the lip 

 of the basin, but at a point in the stream about a quarter of a mile below 

 the outlet at a height of 261 feet, the surface of the loch being 267 feet 

 above sea-level, and the deepest part of the basin being 157 feet. 



Loch Syre, like many of the lochans east of Loch Laoghal, is sur- 

 rounded with morainic deposits. 



NOTES ON THE BIOLOGY OF THE LOCHS OF NORTH SUTHERLANDSHIRE. 

 By JAMES MURRAY. 



Tow-nettings were taken in seven of the lochs. These include three 

 deep lochs (Hope, an Dithreibh, and Laoghal), two very shallow lochs 

 (Chaluim and na Meide), while Loch Naver is intermediate. The 

 biological phenomena are in accordance with those differences, the 

 plankton of the deep lakes being relatively poor, and similar to that of 

 great lakes in general, the shallow lakes having a large admixture of 

 littoral forms. 



All the lochs were rich in algae, especially Desmids, including many 

 of those conspicuous species of western type, alluded to by Messrs. 

 West, which are so characteristic of the extreme north-western fringe 

 of Europe. The northern species of Diaptomus D. laciniatus, D. 

 laticeps, and D. Wierzejskii which are so widely distributed in the 

 north of Scotland, Orkney, Shetland, and the western isles, and which 

 are common in many lochs immediately adjacent, both to the east and 

 south, are absent from most of the lochs of this district. D. laticeps 

 is in Lochs na Meide and Naver, D. laciniatus in Loch na Meide only. 

 D. gracilis is in six of the lochs, and in five it is the only species. 



In the short lists of organisms following the name of each loch, 

 species of general distribution are omitted, only those being included 

 which are interesting on account of their distribution or rarity. 



Loch Hope. Leptodora, Daphnia hyalina (head rounded), Dia- 

 phanotsuma, Fluscularia pelayica, Triarthra lonyiseta, Clathrulina 



