THE AERIAL ALGÆ OF ICELAN[) 395 



N. Icel. 217. L. 218 - W. Iccl. L. 338. 



This small species much rescmbles Xaviciihi Borrichii. but is smaller 

 and with tliinner valves than that species, thc outlinc of the valves being 

 also ditlerent, but evidentl}' it is not far removed from it. Found on 

 mossgrown stones and earth near hot springs (Stora Gja, Helgavatn). 



Naniculæ entoleiæ Cl. 



Navicula contenta Grun. Cleve Syn. I p. 132, N. trinodis Van Heurck 

 Syn. PI. 14, fig. 31a. N. c. var. biceps Cl. Syn. I j). 132. Van Heurck Syn. 

 PI. 14, fig. 31b. 



E. Icel. 7, 14, 24, L. 68, 69. L. 70, L. 85, L. 99, L. 129, 123 — N. Icel. 

 133, L. 160, 169, 179, 215, 217, L. 218, L. 219, 241, L. 253, L. 254 — 

 N. \V. Icel. 268 — W. Icel. 337 — S. Icel. 3;)2 - Vestmannaeyjar 400. 



The typical form of the species frequcntly occurs intcrmixed wilh 

 var. biceps, and all Iransitional forms betwcen them are found. Hence 

 I feel convinced that these two forms ma}' without harm be included 

 within one species, merely representing as they do the cxtremes in the 

 series of variations. In a number of samples 1 have found a form en- 

 tirely destitute of the gibbosities of the val ve, of a rcgular linear form 

 which I provisionally associate with this species. It Nvas present in the 

 foUowing samples: N.Icel. 162 — Vestmannaeyjar L.401, L.403. L.408. 409. 



The species has long been recognised as an aerial species, thus 

 by Van Heurck, Cleve, Boye Petersen, West, Hu stedt. In Den- 

 mark I have found it in several piaces on rock and on mineral ground 

 (Boye Petersen 1915, p. 20). West (1916, p. 421) found it in (piantity 

 as an epiphyte on the leaves of trees in the West Indies, and Hustedt 

 found it in a rock cave (1922, p. 98). Finally Miss Bristol (1920) pro- 

 duced it in cultures in samples of earth that had been dricd up to 

 26 weeks. It would seem then to have considerable ability to withstand 

 desiccation. In Iceland I have found it on more or less damp rocks, 

 on purely mineral ground, aniong mosscs, but only rarely near human 

 habitions. Hence it seems chiefly to thrive well on pure mineral ground 

 where only few organic substances are present. It was particularly 

 common in the samples from rocks, thus in samples 68. 70. 85. 



Navicula perpusilla Grun. Van Heurck Syn. PI. 14, fig. 22, 23. Cl. 

 Syn. I p. 133. Van Heurck Types No. 212, 148. 



E. Icel. L. 24, 68, 69, L. 70, 78, 85, L. 99, 123, 124. L. 128, L. 129 - 

 Vestmannaeyjar 400, 401, 408. 

 Area: Eur., Arct. Amer. 



Curiously enough, Østrup has not mcntioned this species as a na- 

 tive of Iceland. I have, nevertheless, ascertained its presencc in several 

 of the prejjarations examined by Østrup. He must. then, have over- 

 looked it. As regards ils habitat my investigations (|uite confirm what 

 v. Schonfeldt says as to its occurrence: "Zwischen Moosen an uber- 

 rieselten Eelsen«. 12 of the 13 localities mentioncd above are of this 



