FRESHWATER ALG^J. 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. 



NOSTOCACELE. 



. Genus NOSTOC Vaucher. 



58. NOSTOC DISCIFORME sp. n. 

 (PL III., figs. 123-131.) 



Thallo pallide serugiiieo, pellucido, deplanato, adnato vel postea libere natante, 

 forma disci tenuis ovalis vel circularis cum margins levi et regulari, non confiuentis et 

 diametro usque ad 4 mm. ; trichomatibus plus miimsve laxe intricatis, valde tortuosis, 

 vaginis arctis, hyalinis et modice inter cellulas constrictis, ssepe 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 dcpresso-globosis vel rarius 

 globosis, confertis, in catenis dispositis, membrana levi et contentu paulo granuloso ; 

 thallus peridermate distincto, rnucoso, teuui, sed modice tenaci, circumdatus est ; thalli 

 juvenales circulares suut. 



Diam. cell. veg. = 2 5-3 ' 5 /u, ; diam. heterocyst. = 5-6 ' 5 p. ; diam. spor. = 3 ' 5-4 p 

 (an maturae ?) ; diam. vagiuse = 6-7 p.. 



JIab.- 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 P/iormid turn-sheets from the 

 Gap pond, on which it forms small oval or circular transparent discs (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-fitting 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 N. longstaji sp. n. 



