^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.] [Illustration: PLATE IV. Cladophora gracilis. Ulva lanceolata. Ulva lactuca, var. rigida. Enteromorpha clathrata.] ORDER =ULVACEÆ= The plants of this order are, with few exceptions, formed of celled surfaces and show the earliest type of an expanded leaf. The cells form thin membranes, which sometimes are broad surfaces of no definite shape, sometimes are narrow and ribbon-like, or they may be simple or branched tubes. When the membrane [pg055] consists of a single layer of cells it is ^Monostroma^ ("one layer"); when it consists of a double layer it is ^Ulva^; when the layers separate, the thallus becomes hollow, and it is then ^Enteromorpha^. These plants are mostly a brilliant grass-green in color, are silky in texture, and are attached by a small disk to rocks and stones. They abound everywhere, and are commonly known as /green laver/. GENUS ^Ulva^ Frond a thin, silky, flat membrane, sometimes leaf-like, again an extended surface of no definite shape. These are the largest green algæ. They are common everywhere. ^U. lactuca^, the sea-lettuce. Frond a flat membrane of various shapes, sometimes orbicular, again deeply incised, often ribbon-like; margin always much waved or ruffled. In the variety ^rigida^ the frond is oval in outline, not very large, and quite firm or rigid. The species is found on rocks exposed to the action of the waves. (Plate IV.) ^U. latissima.^ Frond a flat, expanded membrane of indefinite shape, but general outline oval, never ribbon-like; attains a size of twelve to twenty-four inches; often deeply lobed, very waved, often perforated with holes; membrane brilliant green, thin, smooth, glossy. It grows apart or in tufts, and is found everywhere, in all stages of growth. It is the largest species of /Ulva/, and is very common on muddy shores. GENUS ^Enteromorpha^