Fucus (Gr.Fucus 700194 a seaweed), a genus of marine melanospermous alga}. Of this genus, which is readily recognized by the inflated air vessels in the substance of the stem or branches, there are but two species upon the Atlantic coast of the United States, two on the Pacific coast, and two upon the coasts of Greenland and Newfoundland. They are found upon rocky shores growing between high and low water marks. Our Atlantic species, fucus vesiculosus and F. nodosus, are popularly called rook-weed and bladder-weed, and form a large share of the vegetation of the tidal rocks from New Jersey northward, where they are conspicuous at low tide and give the rocks a very sombre appearance. Upon the shores of northern Europe the species of fucus are valued as furnishing an important part of the winter fodder of cattle, the animals being regularly driven to the pasturage at the recess of the tide; in some localities these seaweeds are collected and boiled with coarse meal as a food for animals. The chief value of these plants upon our coasts is as a fertilizer, and in some localities large quantities are collected to apply to the land, where they rapidly decompose.

Before the discovery of the process of preparing soda from common salt, the species of fucus were of considerable economical importance, as their ashes, called kelp, were the chief source of soda, and afforded a large income to the owners of estates upon the coasts of Great Britain and Ireland, as well as to the inhabitants of the Orkney, Shetland, and other islands. But little kelp is now produced, as other sources furnish soda more cheaply; but some is still burned for the purpose of procuring iodine, of which the fuci and the related seaweeds are the only available source.-Besides living species of fuci, there are others of particular interest from the occurrence of their fossil remains in the most ancient stratified rocks, associated with those of the oldest forms of animal life, also marine, to which they no doubt served as nutriment. They are abundantly met with in the sandstones of the xYp-palachians, covering the surface of the slabs with irregularly shaped ridges. The flagstones obtained from the Portage group of the New York system so abound with them, that the fossils are seen in every village where these stones are used for the sidewalks.

They are particularly noted in the streets of Geneva, N. Y. (See Hall's Geology of New York," p. 242.) The fossil fuci of the most ancient formations, according to A. Brongniart, are most nearly related to existing species, which belong to tropical climates; but the forms of marine vegetation found fossil in the rocks of the secondary and tertiary formation resemble those now living in temperate climates. - Some species of alga) formerly placed in the genus fucus and others related to it, found about the islands off the southern extremity of South America, are so remarkable as to deserve particular notice. They grow up from deeply sunken rocks, and spread over the surface of the ocean, presenting the appearance of extensively inundated meadows. Ships penetrate with difficulty through the obstructions they present. The stems grow very rapidly, and have been known to attain the length of 700 ft.; Lamouroux describes them as even exceeding 800 ft.; the Agassiz expedition, in the United States coast survey steamer Hassler (1872), found specimens 1,000 ft. long. Dr. J. 1). Hooker, in the " Botany of the Antarctic Voy-age of II. M. Discovery Ships Erebus and Terror, in the Years 1839- 43" gives an interesting account, among others, of the gigantic Lessonia fucescens and macrocystis.

Seen from the surface in sailing over them, they appear like groves of trees, their stems from 8 to 10 in. in diameter, and the branches of the former species spreading out and dividing into sprays, from which the leaves are suspended. Covered with parasitic alga;, and with numerous species of adhering shell fish, as the chitons and patellae, and many Crustacea and radiata swarming among their tangled roots, while fish of different species are seen darting through their foliage, they remind one of the coral reefs of tropical seas. Their stems strewed upon the beaches appear like driftwood, and, as they decay, exhale an almost insufferable odor like that of putrid cabbage. The macrocystis pyrifera is a conspicuous species of the N. W. coast, and is also found in the south Atlantic. It forms stems from 5 ft. to several hundred feet long, which bear pear-shaped air vessels. It is seen upon the beaches rolled up by the waves in great strands larger than a man's body, entangled one with another. The harbors about the Falkland islands, Cape Horn, and Kerguelen Land are so filled with it that boats can hardly be forced through.-The charcoal of fucus vesiculosus, or bladder-weed, has been used in goitre and scrofulous affections.

Its efficacy depends upon the iodine which it contains, although in much less quantity than F. digitatus (or laminaria digitata) and other deep-sea plants. The whole plant has been employed in substance, decoction, and extract, for the purpose of diminishing obesity, and with alleged success. F. (or gigartina) helminthocorton has some reputation in Europe as an anthelmintic, and is said also to be a febrifuge.