1919] SetcheU-Gardner : Myxophyce^e 105 



Thuret, Essai, 1875, p. 382; Bornet and Thuret, Notes Algol. II, 

 1880, p. 163, pi. 40; Bornet and Flahault, Rev. II, p. 344. Rivularia 

 plana Harvey, in Hooker, Brit. Fl., 1833, vol. 2, part 1, p. 394. 



The only two specie.s thus far described are Isactis plana (Harv.) 

 Thuret and /. centrifuga Bornet. Our plants seem to belong to the 

 former and both varieties are represented on our coast. 



Isactis plana var. plana B. and F. 

 Plate 1, figs. 8, 9 



Layer not zonate as a rule; filaments unbranched or nearly so, 

 cohering together. 



On rocks. Puget Sound, Washington, to southern California. 

 Bornet and Flahault, Rev. II, 1886, p. 345. 



Isactis plana var. fissurata B. and F. 



Layer more or less zonate ; filaments pseudoramose, tightly co- 

 hering. 



On rocks. Unalaska, Alaska. 



Bornet and Flahault, Rev. II, 1886, p. 345 ; Setchell and Gardner, 

 Alg. N.W. Amer., 1903, p. 198. 



28. Rivularia Ag. 



Thallus hemispherical, globose or irregularly lobed, at times 

 cavernous, or confluent into a solid expanded stratum; filaments 

 radiating from the center, or from the base, repeatedly false branched, 

 heterocysts basal, some species producing cylindrical spores contiguous 

 to the heterocyst. 



C. Agardh, Disp. Alg. Suec, 1812, p. 43, Syn. Alg. Scand., 1817, 

 pp. xxxviii, 130, 131, Syst. Alg., 1824, p. xix; Roth, Neue Beitr. z. 

 Bot., 1803, p. 261 (pro parte). Cat. Bot., vol. 3, 1806, p. 332 (pro- 

 parte) (not of Roth, Cat. Bot., vol. 1, 1797, p. 212; or Tent. Flor. 

 Germ., vol. 3, 1800, p. 543) ; Bornet and Flahault, Rev. Nost. Het. I, 

 1886, p. 345. 



The status of the name Rivularia was made plain and current by 

 the action of the Third International Botanical Congress at Brussels 

 in 1910, when it fixed the starting point of the nomenclature of the 

 heterocysted Nostocaceae with Bornet and Flahault 's "Nostocacees 

 Heterocystees. " The history of the name, however, is both interesting 



