38 REPORT OF COMMISSIONER OF FISH AND FISHERIES- 



Few genera of alga? have been divided by different, writers into so many artificial 

 and unsatisfactory genera as Rivularia. Some of the described genera are character- 

 ized 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 arti- 

 cle, 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 fila- 

 ments 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 fila- 

 ments 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 

 Glceotrichia, a genus closely allied to, if not to be included in, Rivularia. 



B. atra, Both. (Zonotrichia hemispcerica, Ag. — Euactis amoena, atra, 

 confluens, hemispJicerica, Lenormandiana, marina, Kiitz. — LincTcia atra, 

 Lyngb.— B. atra, Pliyc. Brit., PI. 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, algae, 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. 



B. plioata, Carm., Phyc. Brit., PI. 315. (Physactis, Kiitz.) 



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 &partina roots. Oohasset 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 sur- 

 face. 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 (JSuactis hospita and prorumpens, Kiitz.), which differs from the 



