FRESHWATER, ALG AS. 39 
cells. The heterocysts were single and mostly almost spherical. The only point in 
the published descriptions of the species that could not be verified is the irregular 
widening of the sheaths and their occasional constriction (“vaginis irregulariter 
ampliatis hie et illic constrictis”). The dimensions agree well. 
As far as I am aware this species has hitherto only been observed on wet rocks 
in Italy, by Borzi. 
NOSTOCACE.. 
Genus Nostoc Vaucher. 
58. NosToc DISCIFORME sp. n. 
(Pl. III., figs. 123-131.) 
Thallo pallide zerugineo, pellucido, deplanato, adnato vel postea libere natante, 
forma disci tenuis ovalis vel circularis cum margine levi et regulari, non confluentis et 
diametro usque ad 4 mm.; trichomatibus plus minusve laxe intricatis, valde tortuosis, 
vaginis arctis, hyalinis et modice inter cellulas constrictis, seepe deficientibus ; cellulis 
parvis, plerumque globosis, rarius paulo deplanatis et doliformibus, rarissime ellipsoideis, 
remotis ; heterocystis globosis aut deplanatis aut doliformibus, singulis vel binis vel 
interdum pluribus congregatis, membrana modice incrassata et contentu distincto, 
diametro trichomatis circiter duplo latioribus; sporis depresso-globosis vel rarius 
globosis, confertis, in catenis dispositis, membrana levi et contentu paulo granuloso ; 
thallus peridermate distincto, mucoso, tenui, sed modice tenaci, cireumdatus est; thalli 
juvenales circulares sunt. 
Diam. cell. veg. = 2°5-3°5 w; diam. heterocyst. = 5-6°5 4; diam. spor. = 3°5-4 p 
(an maturae ?); diam. vaginee = 6-7 p. 
/lab.—Gap pond, Winter Harbour; on ice, four feet above frozen watercourse, 
through “ Penknife” ice, McMurdo Bay, September 13th, 1902. 
This characteristic form was very abundant on the Phormidium-sheets from the 
Gap pond, on which it forms small oval or circular transparent dises (fig. 126), which 
may, apparently, later become detached, and exist in a free-floating condition. These 
discs are of all possible sizes, but I have seen none that exceeded 4 mm. in diameter. 
Numerous young colonies were observed and were found to be circular from the very 
first (figs. 123-125) ; the delicate but rather firm investment is immediately developed. 
In the mature thallus the much contorted trichomes (figs. 127, 128, 130) are loosely 
entangled (fig. 131). In rather rare cases a close-fittmg mucilage-sheath could be 
discerned around the trichome (fig. 130). The heterocysts are nearly double the width 
of the vegetative cells and of rather varied shape (figs. 128, 130); most commonly 
they are single or in pairs. The ordinarily subglobose spores are formed in long 
chains (figs. 127, 129). 
For a discussion of the position of this species, see under NV. longstajji sp. n. 
VOL. VI. M 
