THE AERIAL ALGÆ OF ICELAND 411 



According to De To ni this species was found on damp limestone 

 rocks, and v. Sclionf eldt 1913, p. 122,; gives similar habitats. The 

 sample mentioned above was taken from a rocky slope by Siglufjon'iur, 

 bearing evidence that water at times trickled down it. At the time when 

 the sample was taken, it was, however, quile dry. 



Gomphonema parvulum Kutz. Cl. Syn. I, p. 180. V. H. Syn. P1.25fig. 9. 

 E. Icel. 85 - N. Icel. 140 - S. Icel. L. 386, L. 387. 



var. exilissima Grun, Cl. 1. c. Van Heurck Syn. PI. 25, lig. 12. 



S. Icel. 295, 327. 



var. subelliptica Cl. Cl. 1. c. 



S. Icel. L. 386. 



This species, which is very common in fresh water Østrup 160 

 samples), is, like the other Gomphonema species, probably no true aérial 

 alga. De Toni e Forti (1909, p. 22) mention having found it on earth 

 near a hot spring in Africa. Of the above samples those in which it 

 was found with cell contents were taken in very damp localitics, espe- 

 cially No. 386 from the bed of Gljufurholtså which is partly dry in 

 summer, it is true, but where the soil is still quite saturated with water. 



VIII. Cymbelleæ. 



Cymbella Ag. 



Cymbella æqualis W. Sm. Cl. Syn. I, p. 170. C. subæqualis Grun. 

 V. Heurck Syn. Suppl. A. fig. 1. ?Hustedt 1924, Taf. 22, lig. 11. 



E. Icel. L. 14, 85, L.? 113, — N. Icel. 140 - S. Icel. 295?. 



The species is probably spread all over the globe, but it never 

 seems to occur with great frequency. In Iceland it was found in 12 

 samples by Østrup. Thus it is evidently no very pronounced aérial 

 alga. The 3 localities mentioned above are from the immediate vicinit}'^ 

 of houses where there must be supposed to be plenty of nitrogenous 

 substance at disposal. The fourth sample is from a dripping rock, and 

 the fifth from a »formation passagcre«. 



Cymbella incerta Grun. var. naviculacea Grun. Cl. Syn. 1, p. 170. 

 C. naviculacea Cleve 1881, p. 13, Pi. XVI, fig. 1 1 . 



E. Icel. L.? 77, 112, L. 114, 125. 



On the whole this species seems to be rare. Østrup found it in 

 4 samples from Iceland. 11 would seem probable, then, that it is most 

 frequently an aérial alga in Iceland. 3 of the samples given above are 

 from houses and their immediate vicinity (house-wall, stone fence, earth), 

 while the 4th is from naked earth by the roadside. 



