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38 THE MARINE ALGÆ OF NEW ENGLAND. Few genera of algæ have been divided by different writers into so many artificial and unsatisfactory genera as Rivularia. Some of the described genera are characterized by striations or alterations of the sheath which arise from age or unfavorable external conditions. Other so-called genera are characterized by the presence of parasitic plants in the thallus of a true Rivularia. As understood in the present article, the genus differs from Calothrix in the fact that the filaments are imbedded in a mass of jelly and the thallus is of a definite shape and extent. From Isactis, which might be perhaps considered a subgenus, Rivularia differs in having its filaments radiate from a central point instead of being parallel to one another. From Hormactis it abundantly differs in the mode of formation of the filaments. In Rivularia the branches are formed by the division of the filament laterally, the upper part of the branch separating from the main filament, and the two being only in contact at the base of the branch, where a heterocyst is always to be found. In Hormactis the filaments push out sidewise in the form of an inverted V. The apex of the V then elongates upwards and, at the same time, the sides of the V elongate so that, in passing from the interior of the thallus outwards, instead of finding a series of filaments spreading out fan-shaped, we find the filaments converging two by two, which finally unite into single filaments near the surface of the thallus. Moreover, the heterocysts in Hormactis are intercalary, not basal. In none of our marine species of Rivularia have spores been seen, but spores are found in some fresh-water species of Glœotrichia, a genus closely allied to, if not to be included in, Rivularia. R. ATRA, Roth. (Zonotrichia hemispærica, [sic] Ag.—Euactis amæna, atra, confluens, hemisphærica, Lenormandiana, marina, Kütz.—Linckia atra, Lyngb.—R. atra, Phyc, Brit., Pl. 239.) Pl. II, Fig. 2. Thallus solid, globose or hemispherical, varying in size from that of a head of a pin to half an inch in diameter, dark glossy black, filaments straight, .0038-45 mm in diameter, heterocysts about as broad as or rather broader than the filaments, usually somewhat pointed. Var. confluens, flattish, owing to the coalescence of several individuals. Common along the whole coast, on stones, algæ, and stalks of Spartina, often in company with Isactis plana. Distinguished by its dark, shining color and usually hemispherical shape. It is generally minute in size, but occasionally grows as large as a pea or somewhat larger. The variety confluens resembles, to the naked eye, Isactis plana, but is decidedly thicker. Microscopically the two are quite different. R. PLICATA, Carm., Phyc. Brit., Pl. 315. (Physactis, Kütz.) Thallus at first solid, soon becoming hollow, plicato-rugose, folds sinuous, filaments flexuous, .003-4 mm in diameter, heterocysts nearly spherical, about as broad as the filaments. On mud and Spartina roots. Cohasset Narrows, Wood’s Holl, Mass., W. G. F. Common. Although as yet known to occur only at the two above-named localities, this species will probably be found to be common along the whole New England coast, but it is certainly less common than the preceding species. Its favorite habitat is the mud in which Spartina is growing, between tide-marks. It attains a larger size than R. atra, is almost always hollow, and easily recognized by its cerebriformly plicate surface. The substance is softer than in R. atra, the filaments are slightly narrower and less closely packed together, and the heterocysts are rather more spherical than in that species. R. HOSPITA, Thuret (Euactis hospita and prorumpens, Kütz.), which differs from the
REPORT OF THE COMMISSIONER OF FISH AND FISHERIES. 39 preceding species in having filaments .008 mm to .012 mm in diameter, was recognized by Dr. Bornet in company with R. plicata in a specimen from Cohasset Narrows. As we have not been able to recognize the species in any of our own specimens from the same locality, the presumption is that it is not very common. ISACTIS, Thuret. (From ισος [isos], equal, and ακτις [aktis], a ray.) Frond plane, composed of parallel filaments, held together by a tough, gelatinous intercellular substance, ending in a hyaline hair, heterocysts basal, ramifications few. Spores unknown. This genus differs from Rivularia only in that the filaments are parallel to one another so as to form a flat frond, whereas in Rivularia they radiate from a central point and form more or less spherical fronds. It might with propriety be considered a subgenus under Rivularia. I. PLANA, Thuret, l. c. (Dasyactis, Kütz.—Physactis atropurpurea, obducens, Kütz.) Pl. I, Fig. 2. Frond flat, thin, dense, dark green, outline irregular, filaments .0076-95 rmm in diameter, .12-.15 mm high sheaths often torn and striate. Whole New England coast; Europe. Very common on rocks, Fucus, Punctaria, and other algæ, forming dark-green spots, scarcely raised above the substance on which it is growing. HORMACTIS, Thuret. (From ορµος [ormos], a necklace, and ακτις [aktis], a ray.) Frond gelatinous, globose, at first solid, then hollow and plicate, heterocyst intercalary, filaments simple at the surface of frond, bifurcating below. Spores unknown. H. QUOYI, (Ag.) Bornet, in litt. (Rivularia nitida, Farlow, List of Marine Algæ, 1876. Pl. II, Fig. 1. [sic] Fronds gregarious, dark green, plicato-rugose, from a quarter of an inch to two to three inches in diameter, filaments .0028-55 mm in diameter, tortuous, cells of external part of the frond thick and discoidal, becoming more oval in the interior of the frond heterocysts numerous, scattered, about .0038 mm x .0058 mm . Exs.—Alg. Am. Bor., Farlow, Anderson & Eaton, No. 45. Wood’s Holl, Mass., W. G. F.; Falmouth, Mass., Mr. F. S. Collins; Marianne Islands. This interesting species, although it has as yet only been found at Wood’s Holl and. the adjoining coast, will probably also be found at other localities on Long Island. Sound. It grows in considerable quantities upon species of Fucus, at half tide, on the inner side of Parker’s Point, Wood’s Holl, and we have also found it washed ashore on the beaches of Buzzard’s Bay, in the same township. It makes its appearance in June, and disappears in the month of September, being in perfection in the month of July. The fronds sometimes acquire a large size, two or three inches in diameter, but usually they are much smaller and densely aggregated, almost covering the Fucus
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