A filament of ^Polysiphonia^ appears, when seen under the microscope, like a bundle of filaments made up of a central tube, or axis, surrounded by a number of other tubes. It is by the number of these parts, called siphons, which vary in number from four to twenty, that the species is determined. In some plants the siphons are surrounded by a layer of cells, called corticating or bark cells, which give the filaments a solid, uniform appearance. In others the siphons are naked, and the filaments then seem striped or banded with color. It is difficult, without a microscope, to be sure of the classification. There are, however, other characteristics which separate many of the species, and some of these are described below. Two hundred species of /Polysiphonia/ have been named by algologists. The plants are plentiful on all shores, especially in warm, shallow waters. Some species are perennial, but most of them are annuals and disappear in winter. ^P. fastigiata.^ This species grows on /Ascophyllum nodosum/ in a globular tuft, and appears like a dark-brown ball, one inch to three inches in diameter. The frond is a dense mass of rigid filaments branching many times in a forking manner and at broad angles. The ends are of nearly equal length, giving the plant a spherical shape. It is common from New York northward at all seasons, and is easily recognized by its general form and place of growth. Egg-shaped cystocarps, or spore-cases, occur in the ends of the terminal filaments. (Plate XXV.) ^P. nigrescens.^ Frond three to twelve inches long, rigid below, soft above; main stem thick as a bristle, but not always easy to distinguish; branches alternate and densely branched at the ends; siphons twelve to sixteen in number; filaments banded; color black or very dark brown. (Plate XXVII.) ^P. parasitica.^ Frond one inch to three inches long; branched in a pinnate or feather-like manner; filaments flat; all branches and branchlets emanating from the edges and on the same plane, giving a flat frond; color reddish-brown; resembles a fine /Ptilota plumosa/; cystocarps on short stalks. It is found on the California coast. (Plate XXV.) ^P. dendroidea.^ This is a variety of the species /P. parasitica/. Frond four to five inches long; main branches placed at irregular intervals, but the secondary branches at regular intervals and alternate; branches [pg088] set at acute angles, giving the plant a slender appearance; color black or dark brown. It is common on the California coast. (Plate XXVI.)