98 
OSCILLATORIACEiE. 
sometimes coloured, throughout their whole extent, with minute, perhaps microscopic, 
species of this Order. The ocean itself, often for many hundreds of square miles, has 
been found discoloured by microscopic Algrn of -this group, belonging to the genus 
Trichodesmium ; one of which, of a red colour, is found at seasons abundantly in the 
Red Sea, and is supposed to have caused that name to be given to the Arabian Gulf. 
Others are found in the Indian Ocean, one of which is of a bright yellow-green, and 
sometimes deeply tinges the upper strata of the sea off the coasts of Malacca. Others 
have been noticed in equal abundance, but less strongly coloured, in various parts of 
the Pacific, and on the coasts of Australia. 
The species are very difficult to determine, and have been too much multiplied by 
authors : little therefore can usefully be said respecting their geographic distribution. 
Probably, if they are ever carefully investigated, the same kinds will to a large extent 
be found in the most distant localities. This at least seems to be the case with some, 
as Petalonema alatum, Lyngbya majuscula , L. ferruginea , Calothrix scopulorum , and 
many others. 
TABLE OP THE NORTH AMERICAN GENERA. 
* Frond filamentous , branched , olive-brown , in fresh water. 
I. Petalonema. Filaments with a flattened, longitudinally and transversely striate 
sheath, much broader than the endochrome. 
II. Scytonema. Filaments with a cylindrical, hyaline sheath, narrower than the 
endochrome. 
** Frond filamentous. Filaments separate, free, green or purple. 
III. Lyngbya. Filaments very long, flexible, threadlike, bundled together. 
IV. Calothrix. Filaments short, tufted, erect. 
V. Oscillatoria. Filaments rigid, needle-shaped, lying loosely in a mucous matrix, 
usually floating. 
*** Frond formed of numerous parallel filaments contained within a simple or branch- 
ing membranous common-sheath. 
VI. Microcoleus. 
**** Frond formed of numerous radiating filaments invested with firm gelatine ; each 
filament springing from a spherical root-cell. 
VII. Rivularia. 
