1 1 6 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 Carling wark 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 different 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 
