I. Bacillariales (Diatoms). 

 A. Centricæ. 



Melosira Ag., 1824'. 



1. Melosira hyperborea (Giun.) Schult, in Engler & Pranll, Nat. 

 Pllanzeiifam., 1. 1 b, 189(), p. 59; Gran, Norweg. North Pohir Exp., 

 Scientif. Results, vol. 4, No. 11, 1900, p. 52. pi. 3, figs. 11-15. M. 

 niimmuloides, var.'? hyperborea Grunow in V. Heurck, Synopsis, pi. 85, 

 figs. 3—4. 



The species is one of the most characterizing diatoms from the 

 ice-floes. It often occurs in practically pure mass cultures in holes 

 in the ice-floes and in spaces between them. Both in 1906 and in 

 1908 several samples from the pack-ice had this species as their 

 main organism. In most samples from July and August it had 

 resting spores, answering well to the excellent figures in Gran's pa- 

 per quoted above. In October 1906 the samples from Danmarks 

 Havn contained a few empty frustules, and in September 1907 the 

 case was nearly the same, i. e. at that time the vegetation period 

 was over. In June (15*'') 1907 a sample collected in a crack of the 

 land-ice shows us the species in full growth, the chains consisting 

 of frustules in rapid division and with no trace of resting spores. 

 In the later half of July 1908 most of the samples from the coastal 

 water and some from the inner part of the pack-ice contained it, 

 in several of them even as common, and in nearly all samples re- 

 sting spores were present. Thus we get an idea of its life-cycle, 

 as we must suppose that it winters as resting spores, probably frozen 

 in the ice. 



D i strib. Arctic neritic species known from the arctic coast rej»ions 

 and from the inner Baltic. 



Thalassiosira Cleve, 1873. 



2. Thalassiosira Nordenskiöldii Cleve, Bib. K. Svenska Vet. 

 Akad. Handl., Bd. I, No. 13, 1873, p. 6, pi. II, fig. 1 ; Gran, Norske Nord- 



' I have followed the consecutive order of genera and species given in Gran, 

 Nord. Plankton XIX. 



