320 THORODDSEN 



lowing mosses are found on stones in rivulets and brooks: Fonti- 

 nalis antipyretica very frequently, also F. gracilis and F. thulensis, 

 Amblystegium Kneiffii, A. ochraceum, Hypnum rusciforme var. altantica, 

 and others 1 . The vegetation of lakes and pools is much richer 

 and differs considerably according to the depth of the water and 

 the nature of the bottom, etc. The plankton of the Icelandic lakes 

 has as yet been verv little investigated: there are only a few notes 



K/ j ^/ 



to hand from Myvatn and Thingvallavatn. In Myvatn zooplankton 

 only was found; Thingvallavatn contained phytoplankton in which 

 diatoms were dominant 2 . In deep lakes there is usually very little 

 or no vegetation at greater depths, only in places where it is shal- 

 lower does plant-life occur. In Thingvallavatn, however, there are 

 large areas covered with Chara and Nitella, especially at a depth 

 of 13 30 metres, and they extend even down to 38 metres 3 . Where 



ti 



the lakes are shallower various species of Potamogeton and Myrio- 

 phyllum and also Batrachium paucistamineum are common. Near 

 the margin and in smaller pools the most common, and usually 

 dominant, species are the following: Heleocharis palustris, Equisetum 

 limosum , Carex rostrata, Menyanthes trifoliata, and in the southern 

 lowlands Glyceria fluitans is common; to these should be added 

 Hippuris uulgaris, Eriophorum angustifolium, Sparganinm minimum, 

 S. submuticum , Ranunculus hyperboreus and /?. reptans, Subularia 

 aquatica, Callitriche hamulata and C. uerna, Limosella aquatica and 

 several others, the occurrence of which varies somewhat according 

 to the quantity of the water, the conditions at the bottom, etc. 

 Where Equisetum limosum occurs in abundance it is cut annually 

 and used for fodder for milch cows. In Myvaln and in other lakes 

 in Thingeyjarsysla iVos/oc-lumps are found in abundance, often thrown 

 up on the shore in very great quantities. Sometimes in warm sum- 

 mers large areas of Myvatn become turbid; this phenomenon is 

 known by the inhabitants as "leirlos" - it is said to be very injurious 

 to salmon-trout, their gills becoming filled with the fine particles 

 when this occurs they retreat in great numbers to the eastern shore of 

 the lake, where the water is purer and clearer owing to the numer- 

 ous springs which here issue from the lava 4 . 



1 H.Jonsson. 1900, p. 17, 1905, p. 7. 



2 C. H. Ostenfeld and C. Wesenberg-Lund: A Regular Fortnightly Explor- 

 ation of the Plankton of the two Icelandic Lakes, Thingvallavatn and Myvatn. 

 (Proc. R. Society Edinburgh, Vol.25. Part 12, 1906, pp. 1092 1167). 



B. Sjemundsson in Andvari, 1904, p. 89. 



Dr. H.Jonsson informs me that the so-called "leirlos' 1 is probably due to 

 Blue-green Algae perhaps Aphanizomenon /Jos cifjuce. 



