316 JOHS. HOYE PKTKHSKN 



\V. Ic-ol. (ilanimasla^avaln . on stones at Ihc margin, tcmp. 15°. 

 Ve li) 11. 



Area: Eur., As., .MV., X. .\ni Beercn-Eiland. 



The species in (iiiestion seenis lo thrive bcst in rapidly running 

 water, l)iil has also bccn foiind in staj^nanl watcr and on moist earth or 

 nioist rocks. On ils Iceiandic occurrcnce it niostly grew in llie surl-zone. 



Dichothrix compacta Ag.) Born. et IMah. 

 Hornet et Flaliault. Revision I p. 378. 

 Hvcravellir, lemp. 55" C. iWest 1902 p. 243). 



Area: l",ur., X. Am., Færoes. 



Dicholhrix compacta is evidently a species \vhich is not readily dis- 

 tinguished ep. Tilden 1910 p. 277 . This is perliaps the reason why 

 it is so rarelv mcntioned in the literature. 



III. Rivularia (Roth) Agardh. 



Rivularia Biasolettiana Mencgh. i^Lemm. emend.). 

 Lemnierninnn 1910 ]>. 250. 



R. luiimlnld Hornet et Flahault, Revision II p. 348. 

 R. Biasolctliaiia ib. ]>. 352. 



N. Icel. Gasir, on mud ^^v 1899 6. D.)! 

 Area: Eur., X. Am., Alaska, Lappland, Greenland. 



It is but with great reservation that I refer the form in question 

 to /?. Biasolclliana. The tliallus is comi)oscd of 1 2 mm. large spherical 

 tlialli, more or less conlluent, not encrusted wilh lime, rallier soit and 

 easily crushed under cover glass. Trichomes generally 4.4 — 5 /< in diara., 

 at the base, wliich as a rule is somewhat swollen. up to 7.9 // in diam. 

 Lower cells almost quadrate or sliglitly longer than thcir diameter, 

 u|)|)er cells shorter than wide. Heterocysts basal, solitary or in i)airs, 

 8—11 // in diam. Sheaths ycllowish-brown, in the outermost part hya- 

 line, at times distinctly stratiJied and with dilated ochreation at the apex. 

 Threads with sheaths up to 30 // in thickness. Hormogoncs formed in 

 abundance, about 50 /< in length, 4—5 ii in diam. This plant cvidenlly 

 in certain respects resembles R. minulula Kiitz. i Horn. et Flah. var. 0/-- 

 siniana (Men.) Forti (Sylloge Alg. V p. 673); bul in olher respects, such 

 as the aj)pcarance of the thailus, il is dilTerent. 



As the points of dillerence are so insignilicant I should consider 

 it as aimlcss to establish a new S|)ecies on tiie basis of this specimen. 

 The differences consist chielly in the trichomes being thinner mul in 

 the absence of limc-incruslation. Wilh regard to liic lirst jjoint it should 

 bo noled that the trichomes in the Rinnl(iria-s\)ecics probably are con- 

 siderably variable in thickness on the whole, and, as to the second, 

 thai Hornet and Flahault's diagnosis of R. niiiuiliila (1. c.) admits of the 

 thallus being "mollis", i. e. not encrusted with lime. 



Gåsir. wlierc Ihe sample in (pieslion was collecled. is a |)cninsula 

 between Horga and Eyjaljort'^ur, and thcre is Ihus a possibilily ol salt 

 as well as fresh-water. I have tested Ihe adhering, dried mud in order 



