THE FRESH-WATER CYANOPHYCEÆ OF ICELAND 295 



on damp rocks. On the majority of the Icelandic localities mentioned 

 above it grew in running water, in some cases even in streams with 

 rather rapid current. 



Nostoc pruniforme Ag. 



Bornet et P'lahault, Revision IV p. 215, 



N. Icel. Myvatn, Ostenfeld and Wesenberg-Lund (1906 p. 1140) 

 (Nostoc sp., presumably N. pruniforme.). — N. W. Icel. Lake between 

 Brjånslækur and Hagi ^Vt 1915 (H. J.)! — W. Icel. Hveravellir, temp. 

 49^ C. West (1902 p. 244). — S. Icel. Apavatn, washed ashore, spherical 

 colonies up to 3 cm. in diam. ^^/s 1914. Laugarvatn, washed ashore, 

 smaller thalli ^^/s 1914. Ditch at Eyåistjorn near Reykjavikur ^^/e 1896 

 (C. H. O.)! ? Kappastaciavatn in SljettuhH6 ^Vs 1897 (O. D.)! 



Area : Eur., As., Afr., Am., Antarctic, Færoes. 



In the sample from Apavatn trichomes, 6—8 /< in diameter with 

 heterocysts 10 fi in diam., were found in some of the thalli. To estab- 

 lish a new species or variety on account of this seems hardl}^ neces- 

 sary. The form from Hveravellir recorded by West is perhaps not a 

 Nostoc pruniforme at all. West himself denominates it as a var., and 

 neither its thallus nor its habitat agree by any mcans with those typical 

 of the species which generally occurs in larger lakes free-floating or 

 lying on the bottoni. 



Nostoc verpucosum (L.) Vaucher. 

 Bornet et Flahault, Revision IV p. 216. 



S. Icel. Varmå at Reykir in Olfus, temp. 17*^ ^Vs 1914 (2 samples). 

 Area : Ubiquist. 



Apparently always occurring in streams with rapid current. This 

 also applies to the Icelandic locality. 



II. Anabæna Bory. 



The genus Anabæna is as a rule but sparingly represented in the 

 floras from arctic and subarctic countries published up to the present. 

 Of the plankton-forms A. flos-aquæ is almost the only one mentioned, and 

 of the benthos species but a few are recorded. Bachmann (1921 p. 14), 

 however, has in Greenland found 7 species, four of these being plankton- 

 forms. This seems to indicate that in faet a greater number of species 

 occur in these regions than can be seen from the literature. The reason 

 why these species are not recorded in the lists of the species is pro- 

 bable that specimens with well-developed spores are of comparatively 

 rare occurrence, and in their absence any determination is out of 

 the question. 



On basis of dried material it is often very difficult to obtain a di- 

 stinct impression of the appearance of the Anabæna species; in alcohol 

 material, on the other band, the structure of the cells generall}^ stands 

 out beautifully. All the Anabæna species mentioned in the following 

 are found in samples preserved in alcohol or formalin. In the dried 



