274 University of California Puhlications in Botany [Vol. 8 



The genus Percursaria was founded by Bory in 1828 to receive 

 the Ulva percursa Ag. Bonnemaison founded a genus of the same 

 name in 1822 (p. 178) quoted as "Percussaria" by Leman (Diet. 

 d'hist. nat., vol. 38, 1825, p. 425) which is a mixture of filamentous 

 Myxophyceae with no type designated. It certainly seems to us that 

 the Ulva percursa Ag. is a very distinct plant belonging in no other 

 genera of the Ulvaceae as properly limited. We follow Rosenvinge 

 and others in keeping it separate. It is, in reality, a very narrow 

 membrane, seldom, if ever, more than two cells wide. It may possibly 

 be looked upon as a very primitive form among the Ulvaceae. 



Percursaria percursa (Ag.) Rosenv. 



Plate 14, fig. 6 • 



Frond several cm. long, flexuous and contorted, generally irregu- 

 larly and frequently contracted to a single row of cells or expanded 

 to a double row ; cells 10-15/i, wide and from once to twice as long. 



In entangled masses with other filamentous algae in upper tide 

 pools, in ditches in salt marshes and similar places where the water 

 is warmed by the sun. From Alaska (Bay of Unalaska) to central 

 California (San Francisco Bay). 



Rosenvinge, Groenl. Havalg., 1893, p. 963, Alg. Mar. du Groenl., 

 1894, p. 160. Conferva percursa Agardh, Syn. Alg. Scand., 1817, 

 p. 87. Tetranema percursum Areschoug, Phyc. Scand. Mar., Sect. II, 

 1850, p. 418. Diplonema percursum Kjellman, Norra Ishafv. Algfl., 

 1883a, p. 371, Alg. Arct. Sea, 1883, p. 302. Enteromorpha percursa 

 Setchell and Gardner, Alg. N.W. Amer., 1903, p. 214; Collins, Green 

 Alg. N. A., 1909, p. 197; Collins, Holden and Setchell, Phyc. Bor.- 

 Amer. (Exsicc), no. 968. 



This interesting plant, so unlike a member of the Ulvaceae in 

 general appearance, seldom occurs pure, but is generally mixed with 

 those species of Cladophora, Enteromorpha and filamentous M^^xo- 

 phyceae which delight in the same conditions of life. It is readily to 

 be told from its associates by the very symmetrically placed double 

 row of cells. 



