THE FRESH-WATER CYANOPHYCEÆ OF ICELAND 293 



examined the sample in Phycotheca Boreali-Aniericana nr. 307 myself 

 and iirrived at the same conclusion. 



M. Stecnstrupii is distinguislied from M. subtornlosns (Bréb.) Gom. by 

 (1) tliinner tricliomes, (2) cells longer than the diameter, (3j distinct, not 

 diffluent sheaths, and (4) the apical cell being apparently of a much 

 more pronounced conical form judging from the figures in Gomont. 



M. Sleenstriipii grew at Reykir together with Scytoncma mirabile, 

 Phormidinm Trcleasei as well as Stigonema sp. on stratified silicioiis sinter. 



In the sample from Laugarne at Reykjavik it occurred. inter alia, 

 in Company with Bryiim argenleum. In this sample the chlor-zinc-iodine 

 reaction took cffect more slowly and did not at all attain such a rich 

 colouration as in the sample from Reykir. 



At Gullfoss it grew on earth among Collema sp. in piaces moistened 

 by the fine spra}^ from the waterfall. 



In spite of the faet that this species in two piaces has been found 

 at hot springs it is hardl}' a true thermal alga, for in that case it would 

 presumably not have been able to thrive at Gullfoss. It is rather the 

 constant supply of moisture — at the hot springs in the form of vapour, 

 at the waterfall in the form of the fine spraj^ — which conditions its 

 vigorous growth. 



II. NOSTOCAGEÆ. 



I. Nostoc Vaucher. 



The determination of the Nostoc species often offers great difficulties, 

 nay, is frequently impossible to undertake. A safe determination is espe- 

 cially conditioned upon the presence of spores; but these are in the 

 raajority of species of rare occurrence. In a great number of samples I 

 have therefore found Nostoc thalli which were impossible to determine. 

 Some uncertainty has even been unavoidable in the following deter- 

 minations. 



Nostoc carneum Ag. 



Bornet et Flahault, Revision IV p. 196. 



S. Icel. Ytri Skogar ^V? 1901 (H. J.)! Drangshliå ^o/^ 1901 (H. J.)! 

 Gljufuiholtså on stones, light current, temp. 15° ^^/s 1914. — E. Icel. 

 Sto5 12/8 1894 (H. J.)! 



Area : Eur., Afr., Am., Færoes, Alaska. 



According to literature it has most frequentlj' been found in stag- 

 nant water, attached as well as free-floating. In a single case it is re- 

 corded as growing on dripping rocks. 



Nostoc muscorum Ag. 



Bornet et Flahault, Revision IV p. 200. 



W. Icel. Hveravellir, temp. 55 « C. West (1902 p. 244). 

 Area: Eur., As., Afr., Am., Alaska. 



