THE AERIAL ALGÆ OF ICELAND 367 



liere enumerate the species found , l)ut refer the reader to Ihe pre- 

 ceding lists (p. 343). This association of Diatoms does not seeni to 

 show any special chaiacterislics; it consists of a mixture of the usual 

 terrestrial and hydrophilous species. Allogether I do not think Ihat 

 speciall}' thermophilous Diatom species occur. 



4. Temporary Algal Associations. 



By »Formations passagéres« Gomére (1913, p. 5) means algal 

 associations living in puddles, ditches, and similar piaces where the 

 amount of water is so small that it entirely disappears in the dry 

 summer time. The algæ growing in such piaces can hardly be in- 

 cluded among the aérial algæ, since they are quite submerged during 

 growth. When the water evaporates, some of them will be ahle to 

 form resting stages that wåll survive desiccation, others die away 

 with the e.Kception of a few individuals that have found shelter 

 below other algæ which protect them against evaporation, others 

 again perhaps die out entirely. The vegetation in these piaces will 

 therefore be variable, and you cannot count on finding the same 

 species in the same waterhole each year. What species develop 

 will depend upon which species survive the preceding period of 

 desiccation, and what germs have happened to be added in the 

 moist period. Hence it cannot be expected that the few samples 

 I have coUected from this kind of localities in Iceland should give 

 a complete picture of this algal association. I shall not, therefore, 

 attempt to describe the characleristics of the algal flora but refer 

 the reader to the lists of species in the preceding summary of the 

 individual localities. The samples of »formalions passagéres« are 

 133, 140, 302. 



SYSTEMATIC ENUMERATION OF THE SPECIES 



OBSERVED. 



CYANOPHYCEÆ 



(SEE THE PKECEDING PAPER). 



DIATOM AC EÆ. 



The Diatoms live principally in the top layer of the earth or 

 among the mosses and algæ on the ground. 1 therefore scraped 

 the uppermost thin layer olT the samples of earlh collected. A little 



