98 OSCILLATOEIACE^. 



sometimes coloured, throughout their whole extent, with minute, perhaps microscoi)ic, 

 species of this Order. The ocean itself, often for many hundreds of square miles, has 

 been found discoloured by microscopic Algai of this group, belonging to the genus 

 Trichodesmium ; one of which, of a red colour, is found at seasons abundantly in the 

 Eed 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 oif 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. 

 Proba1)ly, 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, Li/nijhya majuscula, L. ferrug'mea, Calothrix scopulorum, and 

 many others. 



TABLE OF THE NORTH AMERICAN GENERA. 



* Frond filamentous^ branched^ olive-hroivn, 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 icith firm gelatine ; each 

 filament springing from a spiherical root-cell. 



VII. ElVULARIA. 



