The silkweeds. This order is characterized by cylindrical cells strung end to end, forming threads or filaments, branched and unbranched. The plants inhabit both fresh and salt water, and are very abundant and widely distributed. They grow in dense tufts, often matted at the base. GENUS ^Ulothrix^ A yellow-green, unbranched, decumbent, soft, hair-like fleece on the surface of rocks, extending indefinitely. This genus differs from /Chætomorpha/ in the character of its filaments, which are soft and gelatinous in ^Ulothrix^, but bristle-like and wiry in /Chætomorpha/. GENUS ^Chætomorpha^ The frond is filiform; the filaments are coarse, rigid, and unbranched. In some species the filaments grow straight and in tufts from a definite base; in others they are twisted together and are prostrate. Often they are found floating in masses. In ^C. tortuosa^ the filaments are as fine as human hair, but rigid, and so closely interwoven as to resemble a layer of wool on the rocks. The cell-divisions give a striped appearance to the filaments when dry. ^C. melagonium.^ This species is dark green, with filaments erect, coarse as a double bristle, and wiry; five to twelve inches long. It is found in rock pools from Boston northward. It does not adhere to paper in drying, and loses its color if immersed in fresh water. (Plate III.) ^C. ærea.^ Yellowish-green, with filaments erect and less rigid than [pg054] in /C. melagonium/, which it otherwise resembles; tufts three to twelve inches long; cell-divisions very marked. This species is found in rock pools from Cape Cod to New York Bay. ^C. linum.^ Bright green; filaments coarse, rigid, twisted together, and prostrate. It is found floating in masses and forming strata on rocks and gravel from New York northward. It is thought by some that this, as well as ^C. picquotiana^, which it closely resembles, is not a true species, but consists of the mature plants of the species /C. melagonium/ and /C. ærea/, which have become detached from their holdfasts and have continued to grow. (Plate III.) GENUS ^Cladophora^ ("/Branch-bearing/") Frond filiform, branched. There are many species of ^Cladophora^, which differ from one another in their branching, color, and size. They abound on rocks at low-water mark, in tide-pools, in muddy ditches, and on wharves. They are especially characterized by being so profusely branched as to form tufts or spherical masses, by which the collector can easily distinguish the genus. ^C. arcta.^ Bright, glossy green; filaments fine, erect, much branched, two to eight inches long; tufts dense, more or less entangled, and in bunches, giving a starry effect. The species is common on rocks near low-water mark from New York northward. The plants vary slightly in appearance with the season. (Plate III.)