THE GREEN ALGAE OF NORTH AMERICA 209 



Spongonwrpha arcta, Bangia fusco-purpurea, Hormistia, etc. In 

 all specimens from the Pacific coast that have been examined, the 

 cells are decidedly smaller than in specimens from the Atlantic ; 

 8-10 fj. diam. in the former, 12-16 //. diam. in the latter, seen 

 superficially. 



2. M. GREVILLEI (Thuret) Wittrock, 1866, p. 57, PL IV, 

 fig. 14; Farlow, 1881, p. 41 ; P. B.-A., No. 15 ; Ulva Laduca 

 Harvey, 1846-51, PI. CCXLIII ; 1858, p. 60. Frond attached, 

 at first saccate, then opening at the top, and ultimately 

 splitting to the base ; soft and delicate, pale green ; membrane 

 15-20 (* thick, cells quadrate with rounded angles, closely set ; 

 in cross section horizontally oval, 12-14 !" high- Sporiferous 

 cells enlarged, vertically elongate in cross section ; cell wall dis- 

 solving after emission of spores. Greenland to N. J.; Alaska 

 to Cal. Europe. 



The saccate form is plain in young plants, and may persist for 

 some time when growing in still water ; but at exposed points 

 the frond is soon torn open, and in mature plants all trace of 

 the original shape is lost, wherever growing. 



An early spring plant except in Arctic regions, where it con- 

 tinues till August. 



Var. VAHUI (J. Ag.) Rosenvinge, 1893, p. 949; M. Vahlii 

 J. G. Agardh, 1882, p. 109, PI. Ill, figs. 84-89. More slender 

 in form, often cylindrical, retaining its saccate shape longer, 

 and with cells arranged in more or less distinct longitudinal 

 series. An early spring plant. Greenland to Mass.; Alaska. 



Var. lubricum (Kjellman) nov. comb.; M. lubricum Kjellrnan, 

 iSyya, p. 48, PI. IV, figs. 8 and 9. Frond up to 15 cm. long, 

 pale or whitish green, delicate, very lubricous and flaccid, of 

 irregular outline, laciniate, plicate, margin often crisped and 

 lacerate; frond 18-22 /A thick; cells seen superficially circular 

 or rounded angular, often in twos or fours, cell wall thick ; in 

 cross section horizontally ovate or oblong, 4.5-8 p. high. Green- 

 land, Alaska. Northern Europe. 



This has never been found attached, but always as shapeless 

 floating fronds ; with our imperfect knowledge of it, it is prob- 

 able that it is a form of M. Grevillei in which the cells are some- 

 what shrunken, the membrane much gelatinized. 



3. M. LACTUCA (Roth) J. Ag., 1882, p. 102, PI. Ill, fig. 90; 

 Areschoug, Alg. Scand. Exsicc., No. 121; P. B.-A., No. 

 1271. Frond at first an elongate, obconical sac, soon splitting 

 to the base into linear, simple or forked laciniae, usually with 



