THE AÉRIAL ALG.K OF ICELAND 415 



As will appear from the list above of the samples in which it was 

 found, this form is distributed throughout the island and occurs on the 

 ground, in almost all kinds of localities, most frequently, however, among 

 Prasiola and mosses. It has also been found on walls at only a small 

 height above the ground. 



Hantzschia amphioxys var. genuina drun. Grunow, Die Diat. Fr. 

 Jos. Ld. 1884, p. 47. 



E. Icel. 41, 61, L. 68, 131 - N. Icel. 218 - N. \V. Icel. L. 269 — 

 W. Icel. L. 307 — S. Icel. 295 — Vestmannaeyjar L. 401. L. 408, 409. 



This variety is rarer in Iceland than var. xcroj)hil(i, in Company 

 with which it often occurs. From the material at band it has not been 

 possible to infer what life condilions the two forms respcctively require. 



Nitzschia Grun. 



Nitzschia amphibia Grun. V. Heurck Syn. PI. 68, fig. 15,16. Cl.etGrun. 

 Arct. Diat. p. 98. 



E, Icel. 7, 8, 14, 24, L. 25, 28, 41, 77, 78, 85, L. 92, L. 99, 112 — 

 N. Icel. L. 135, 140, 173. 215 - W. Icel. 321, 337 - S. Icel. 294. 



The species is common in fresh water (Østrup 55 samples from 

 Iceland , nor is it evidently uncommon on land. where I have often 

 found it on rocks with trickling water and on rather damp ground near 

 the edges of ditches. I have reason to believe that it will not withstand 

 much desiccation, whereas it seems very tolerant of organic matter in 

 the substratum sample No. 173, »soil saturated with urine«) and capable 

 of thriving on purely mineral ground. 



var. acutiuscula Grun. V. Heurck Syn. PI. 68, fig. 19-22. Cl. 



et Grun. Arct. Diat. p. 98. 



N. Icel. 162. 



Only one specimen observed. 



Nitzschia Clausii Hantzsch. Grunow 1878 p. 119. Van Heurck Syn. 



PI. 66, fig. 10. 



E. Icel. L. 7, L. 133 - N. W. Icel. L. 264 — S. Icel. 386 — Vestmanna- 

 eyjar 410. 



I have sometimes found somewhal dcnser carinal dots than recordcd 

 by Grunow, viz. 14 inlO/<. N.aiicjuillula Schum. can hardly be distinguished 

 from N. Clausii. 



Østrup having found the species in 14 samples from fresh water, 

 it can hardly be said to be a pronounced aérial species. I found it in 

 the largest quantity in the samples from roads and their vicinily 133, 

 264) where there will always be some manure. The same will be the 

 case with sample 410 from a bird-cliif. It would, however, be premature 

 to draw wider conclusions from this faet. 



