THE AÉRIAL ALGÆ OF ICKLAND 363 



N. peregrina v. Menisciis, and Nitzschia vilrca v. salinariim. In anolher 

 cave (402, 403) of the same kind Ihe floor was formed of calcareous 

 sand consisting of fragments of moUusc shells, Bi yozoa etc. on whicli 

 grew a sterile Vaiicheria. The walls of the cave \vere covered with 

 a Diatom vegetation of the same character as that of the first- 

 mentioned cave. Of more or less halophilous species there occurred 

 here Naviciila jjiisilla, N. cincta and Nitzschia vilrea \. salinariim. 



In a cave in the northern part of Heimaey Helgi Jonsson 

 fonnd a vegetation of a land Floridea in 1897. This was described 

 by Rosen vinge (1900, p. 61) under the name oi' lihodochorton is- 

 landiciim. Later Helgi Jonsson (1911, p. 119) gave furlher parti- 

 cuiars of its habitat to which 1 refer the reader. 



d. Loose Stones. 



On stones not connected with the firm rock, algæ are often 

 fonnd, but at the outset it is probable that the algal vegetation of 

 such stones will vary greatly according to their situation. 



On the Icelandic farms there are occasionally stone fences, for 

 insiance round the tun (homefield). On the stones of a couple of 

 such fences (113, 353) I have found Prasiola fiirfiiracea together with 

 a number of Diatoms, common to both were Pmmilaria borealis and 

 P. intermedia. In the town of Reykjavik the vegetation on the stones 

 was of quite a difTerent character, there were several Chlorophyceæ 

 and no Diatoms. In samples 270 and 275 were found Desinococciis 

 viilgaris, Pleiirococcus viilgaris, Prasiola crispa, and Trochiscia hirta 

 (compare remarks above on the algal vegetation on woodwork at 

 Reykjavik (p. 347)). 



On a large stone in the cleft »Stora Gjå« near Mvvatn there 

 grew mosses, Prasiola crispa, Pleiirococcus viilgaris, Trochiscia hirta, 

 besides numerous Diatoms. Aboul the conditions in Stora Gjå I 

 refer the reader to my remarks above (p. 349). A hot spring has 

 its outlet at the bottom of the cleft. Its vapours will no doubt keep 

 the air in a constant state of moisture and this in connection with 

 the sheltered position will favour the development of aérial algæ. 



On gravel and loose stones of volcanic origin I have found algal 

 associations in two piaces where their occurrence seemed to me 

 very peculiar. I shall describe these occurrences in more detail 

 without attempting to explain them. 



On the edge of a small volcanic crater (185) I found Prasiola 

 crispa, Diatomella Balfouriana, Hantzschia amphioxijs, Pinniilaria 



