182 KJELLMAN, THE ALGA OF THE ARCTIC SEA. 
sporangia. Among its forms f. lapponica approaches most nearly to A. boreale by the 
branches of the last and next to last order being more generally one-sided, by the 
systems of short branches being spreading or recurvate, and by the branches of the 
last order being coarser and stiffer. The typical form of the species, according to my 
opinion, is that which has the branches of the last and next to last order longer and 
more slender, rarely one-sided, but sometimes opposite, sometimes alternate, or some 
being opposite or alternate, others one-sided. When now and then they are one-sided, 
they occur on the inside as well as the outside of their main axis. This form stands 
very near A. americanum, as has been correctly stated by Gozi. The principal diffe- 
rences between them are as follows: A. americanum is larger and more tufted, more 
violet in colour, with longer cells even ten times as long as thick — and fewer, 
longer, and more flaccid, branch-systems of the last order, with longer and finer side- 
branches. The form corallina differs from the typical form by having branches and 
branch-systems densely crowded into dense, button-shaped fascicles at the tops of the 
main axis and the long branches, and by four branch-systems issuing from most 
of the articular cells of the main axis and the long branches. In these respects 
it approaches A. cruciatum, from which it differs by the longer cells and the finer, 
more flaccid and elongated branches of the last order, and by the tetrasporangia being 
situated not at the base of the secondary branch-systems supplying their branches of 
the first order, but on the branches of the first order representing the axes of the 
second order of these branch-systems. 
From the supposition that A. plumula belongs properly to the Atlantic and the 
Mediterranean and that it occurs, as he thinks, less frequent and luxuriant in the Arctic 
Sea, Gost concludes that this species has immigrated into the Arctic Sea and become 
strongly changed there. It is impossible, of course, to determine with certainty how 
this has been, but for my own part I should be more inclined to adopt a quite contrary 
opinion, that is to say, that A. boreale has originated within the Arctic Sea and that A. 
plumula and other species have issued from it and been developed in a southward direction. 
A. boreale is so widely distributed in the Arctic Sea, that it can hardly be assumed to be 
an immigrant. I have found it commonly diffused in all the parts of the Arctic Sea 
that I have investigated. It is, indeed, often but little luxuriant and seldom occurs in 
greater numbers; but | have pointed out above and already mentioned in Spetsb. Thall. 1, 
p. 27 that this is not always the case. Touching A. plumula, it may be remarked that is often 
difficult to arrive at any certain knowledge about the frequency of a species bymeans of 
the terms generally used, particularly with regard to the occurrence of the species in 
question I must admit that I know but very little. ArescuouG states that on the coast of 
Scandinavia it is »minime infrequens». On the ground of my own experience, I should trans- 
late this expression by saying that, like A. boreale, it is commonly diffused, but seldom 
or never appears in greater masses nor surpasses the last-mentioned species in luxuriancy 
of growth when this is most luxuriantly developed. On the coasts of Britain the present 
species according to Harvey is »not uncommon», which need not mean anything else 
than that it is rather commonly spread. It is rare on the north-west coast of France 
at Cherbourg according to Le Joxts, and on the north-east coast of America according 
