_Nostocaceae._ Thallus consists of simple or branched rows of cells in which special cells known as _heterocysts_ often occur. Reproduction by means of germ-plants or _hormogonia_, or by resting cells specially modified to resist unfavourable conditions. In both families the individuals are surrounded by a gelatinous envelope, which in some genera assumes the form of a conspicuous and comparatively resistant sheath. Marine, freshwater, and aerial forms are represented among recent genera. Several species occur as endophytes, living in the tissues or mucilage-containing spaces in the bodies of higher plants. In addition to the frequent occurrence of blue-green algae in freshwater streams and on damp surfaces, certain forms are particularly abundant in the open sea[176], and in lakes or meres[177] where they are the cause of what is known in some parts of the country as “the breaking of the meres” (“Fleurs d’eau”). From the narrative of the cruise of the Challenger, we learn that the Oscillariaceae are especially abundant in the surface waters of the ocean. The “sea sawdust” so named by Cook’s sailors[178], and the same floating scum collected by Darwin[179], affords an illustration of the abundance of some of these blue-green algae in the sea. Another manner of occurrence of these plants has been recorded by different writers, which is of special importance from the point of view of fossil algae. On the shores of the Great Salt Lake, Utah, there are found numerous small oolitic calcareous bodies thrown up by the waves[180]. These are coated with the cells of _Glœocapsa_ and _Glœotheca_, two genera of the Chroococcaceae. Sections of the grains reveal the presence of the same forms in the interior of the calcareous matrix, and it has been concluded, on good evidence that the algae are responsible for the deposition of the carbonate of lime of the oolitic grains. By extracting the carbonic acid which they require as a source of food, from the waters of the lake, the solvent power of the water is decreased and carbonate of lime is thrown down. In similar white grains from the Red Sea[181] there is a central nucleus in the form of a grain of sand, and cells of Chroococcaceae occur in the surrounding carbonate of lime as in the Salt Lake oolite. Prof. Cohn of Breslau in 1862 demonstrated the importance of low forms of plant life in the deposition of the Carlsbad “Sprudelstein[182].” On the bottom of Lough Belvedere, near Mullingar in Ireland[183], there occur numerous spherical calcareous pebbles, of all sizes up to that of a filbert. From a pond in Michigan (U.S.A.)[184] similar bodies have been obtained varying in diameter from one to three and a half inches. In the former pebbles a species of _Schizothrix_, one of the Nostocaceae occurs in abundance, in the form of chains of small cells enclosed in the characteristic and comparatively hard tubular sheath, and associated with _Schizothrix fasciculata_ there have been found _Nostoc_ cells and