42 F. E. FRITSCH. 
fuscescentibus vel hyalinis, non lamellatis; cellulis parvis, globosis vel interdum 
doliformibus, arcte vel laxe dispositis, contentu dilute srugineo, paulo eranuloso ; 
heterocystis pleramque non in vaginis inclusis, elobosis aut interdum modice elongatis, 
etiam ellipticis finibus paulo acutis, diametro articuli vegetativi circiter duplo latioribus, 
plerumque singulis ; sporis ignotis. 
Diam. cell. veg. = 3-4; diam. heterocyst. = 5-7; long. heterocyst. = 6-7°5 p. 
ITab.—Gap pond, Winter Harbour. 
This characteristic species was very obvious, owing to the dark colour of even 
relatively small colonies. The latter were often completely spherical (fig. 139), 
although young colonies of an elongated shape were not uncommon (figs. 143, 144) ; 
in the older colonies, owing to the central part becoming hollow, the thallus often 
appears flattened and may even become torn open and rather irregular. As no spores 
were observed, it is conceivable that the thallus may attain even larger dimensions 
than those given in the diagnosis. 
The most striking feature lies in the highly developed sheaths of the trichomes 
(figs. 140-142). They are excellently developed in the young colonies, but are 
generally quite distinct also in the outer parts of the older colonies. The sheaths are 
sharply differentiated into two regions, an outer one, which is diffluent and hyaline or 
pale yellowish-brown, and an inner one, which is denser, sharply demarcated from the 
outer and of a brown or deep blackish-brown colour. This inner sheath shows very 
obvious constriction between the cells, often amounting to complete septation (figs. 139, 
141, 142). In the inner parts of older colonies there is no differentiation in the 
sheath. 
Although no spores were observed, young stages of this species were not 
uncommon. They consist of a rather dense tanele of filaments, already provided with 
well-marked brown sheaths (figs. 139, 144). Quite small colonies already possess 
an investment. 
L. Gain has recently (“ Deux especes nouvelles de Nostoc provenant de la région 
antaretique sud-américain,”’ Comptes Rendus, cli. (1911), pp. 1693-4) described a new 
species of Nostoc under the name of NV. pachydermaticum, based on a single specimen. 
It is possible that NV. fuscescens is identical with Gain’s species, but there are certain 
striking differences. Thus Gain speaks of “articulis ellipticis, 2°5-3 4  crassis, 
4—5 » longis,” whereas the cells of NW. fuscescens are nearly or quite spherical ; the 
sheaths are described as ‘‘ amplis, mesentericis, fuscescentibus, 13-16 » crassis,” but 
there is no mention of their characteristic differentiation which, as Gain evidently had 
a very small colony before him, would have been very obvious. For these reasons I 
scarcely think that the two forms can be identical, and have preferred to describe the 
specimens from the Gap pond as a distinct species. 
N. fuscescens will probably find its place in Bornet and Flahault’s fourth section 
Intricata, most of the species of which are spherical at first and have coloured sheaths, 
although none show the striking differentiation characteristic of the Antarctic species, 
