^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.)
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.)
^C. rupestris.^ Dark green; filaments straight, rigid, tufted; branches crowded; many branchlets flattened against the filaments, so that the alga somewhat resembles grass; five to ten inches long. The cell-divisions show plainly. Plants of this species do not adhere to paper in drying. They are found on rocks at low-water mark, and are common on the northern New England coast. (Plate III.)
^C. gracilis.^ Bright yellow-green; filaments very fine, loosely tufted, three to twelve inches long, soft, silky, much branched; branches rather short, and branchlets more or less curved and arranged in a comb-like manner. It grows on wharves, in muddy pools, and on eel-grass. (Plate IV.)
[Illustration: PLATE III. Chætomorpha melagonium. Chætomorpha linum. Cladophora arcta. Cladophora rupestris.]