116 Proceedings of the Royal Society of Edinburgh. [Sess. 



slightly peaty, the water is clear and bright, so that vegetation at the 

 bottom may be observed through a depth of 10 feet. In many places the 

 stones, etc. from the bottom were thickly incrusted with the green sponge, 

 Spongilla fluviatilis, and some of the sandy areas were abundantly strewn 

 with the large mussel Anodonta cygnea. The Dee is, indeed, a particularly 

 favourable habitat for this mollusc, and some of the country folk add a few 

 pounds annually to their incomes by the sale of the pearls which they 

 frequently find in them. Subsequently, when visiting Carlingwark Loch at 

 Castle-Douglas, I found the sandy-muddy bottom covered in places with 

 enormous quantities of this mussel, some specimens being 7 inches long. 

 Directing the attention of my boatman to this fact, he determined to take 

 the first opportunity for a pearl hunt, having learned the method of search 

 from a resident in the neighbourhood of Loch Stroan, and incited also by 

 the knowledge that the latter had sold a few pearls for £6. His chance 

 soon arrived, as I had perforce to give him a holiday, because I became hors 

 de combat from the stings of insects, chiefly clegs, which caused such swell- 

 ing to my face that I was almost blinded for two or three days. Repairing 

 to the loch, hundreds of mussels were brought ashore by this " worthy," but, 

 to his utter disgust, after a whole day's labour, not a single pearl was found. 

 It may be that the diff"erent conditions prevailing in Carlingwark Loch do 

 not readily induce the formation of pearls in the bivalves. 



The bottom of Loch Stroan is to a great extent sandy or muddy, but no 

 living vegetation occurs at a greater depth than 20 feet, as one might expect 

 would be the case from a consideration of the clearness of the water. The 

 reason for this is that, beyond depths of from 15 to 20 feet, the loch-bottom 

 is covered with the remains of grass-like moorland and marsh vegetation, 

 chiefly those of Molinia, Carex, and Scirpus, brought into the loch by winter 

 floods, as at Loch Doon (p. 101). The dead remains do not come so near the 

 surface here as at Loch Doon because of the scour caused by the River Dee 

 in flood-time. In this case the bulk of such material is derived from the 

 flat, marshy ground, extending, as already mentioned, from the west shore 

 of the loch. Fig. 24 shows a great bank of this dead substance deposited by 

 winter floods high above the normal water level. 



The principal plants at this loch are as follows : — Littorella lacustris, 

 Lobelia Dortmanna, Subularia aquatica, Isoetes lacustris, Apium inundatum, 

 Myriophyllum alterniflorum, Scirpus fluitans (fig. 25), Potamogeton poly- 

 gonifolius, P. natans, Sparganium natans, Glyceria fluitans, Fontinalis 

 antipyretica, Phragmites communis, Heleocharis palustris, Equisetum 

 limosum, Juncus effusus, Carex rostrata, C. Goodenovii, C. flacca, and 

 Ranunculus Flammula, all the foregoing being abundant. Callitriche 



