THE AERIAL ALGÆ OF ICELAND 391 



only transition forms between f. Cohnii and f. Goppcriiana in some few 

 samples. I liave mentioned f. elliptica in 1915, p. 287; 1 have not found 

 it in Iceland. 



Of the remaining forms the following, wliich shall now be more 

 closely described. were found by me in Iceland. 



Navicula mutica Kiitz. f. Cohnii Hilse. Clcve Syn. I, p.l29. Van Heurck 



Syn. PI. 10, lig. 17. Van Heurck, Types No. 119. ' 



E. Icel. 8, L. 40, L. 68, L. 69, L. 77, L. 78, L. 92, 99, L. 112, L. 113, 

 L 114, L. 121, L. 129 - N. Icel. L. 135, L. 137, 140, 160, L. 162, 169, 

 L. 173, L. 179, L. 184, L. 196, 217, 2l8, L. 219, L. 241, L. 242, 252, 

 L. 253, L. 254 — N. W. Icel. 268, L. 269 - S. Icel. 295, L. 381 - Vest- 

 niannaeyjar 400, L. 403, L. 407, L. 408, L. 409. 



This form is characterised by its elliptic-lanceolate oftcn slightly 

 rostrate valves with rounded apices. As shown in Van Hcurck's figure 

 (1. c), the valve is as a rule distinctly asymmetricai, the side on which 

 the isolated point is found being distinctly smaller than the other one. 

 The punctation is rather variable, sometimes the puncta are very coarse 

 (samples 408, 409, forms that approach N. miiticopsis Westi, sometimes fine. 

 Bristol shows various punctations ^1920, p. 68 and p. 69, lig. 9,2, 3, 6l 

 in a number of different specimens which must most probably be re- 

 ferred to f. Cohnii. 



This form, as well as other forms of N. mutica, is stated both by 

 Van Heurck (Traité p. 206) and Cleve (1. c.) to live chielly in brackish 

 water. Judging by my experience this is hardly quite correct. They 

 are more probably all of them pronounced aérial diatoms, with the cx- 

 ception, perhaps, of some of the undulate forms which I have not had 

 an opportunity of observing. The form Colinii is extremely widespread 

 in Iceland and most probably over the greater part of the globe, on 

 the surface, and I have found it in Iceland on very diverse substrata, 

 but most frequently on the ground near human habitations, both in the 

 towns and near the farms, on the turf of which the older houses are 

 built, on fences round the tun, etc. In such piaces I have often found 

 it in very great quantity in the samples. This agrees well with my ex- 

 perience from Denmark Boye P. 1915, p. 287\ where I have found it 

 on the manured soil both of gardens and lields as well as on a jmrely 

 mineral substratum. In England Miss Bristol has likewise found the 

 species in samples chielly of arable soil. It would seem, however, that 

 the species thrives best on a substratum abounding in organic and more 

 especially nitrogenous substances. This form of the sjK'cics was found 

 by Moore and Carter (1926, p. 118) in samples of soil taken from a 

 depth of down to 4 feet. 



f ventpicosa Kiitz. Cl. Syn. I, p. 129. Meister 1912, Taf. XIX, 



hg. 16. 



N. Icel. L. 137, 217, 218. 



The specimens I have seen from Iceland bear most resemblance to 

 Meister's figure cited by me, but are somewhat longer and tluis ap- 



26* 



