1903] Collins, — The Ulvaceae of North America 13 
distinct from M. Grevi//ei, or at most a variety characterized by the 
slightly thicker frond, with cells occupying considerably less than 
half the thickness of the frond in cross section. The species is com- 
mon all along the Atlantic coast from Greenland (March to August) 
at least as far south as New Jersey (spring months), and has been 
found on the Pacific coast from Monterey to Alaska. It occurs 
throughout northern Europe. Farlow (4) 41. Pike (1) 106. 
Dame & Collins (1) 157. Martindale (1) 92. Rosenvinge (1) 946, 
fig. 50; (2) 149, fig. 50; (3) 117. Collins (8) 44. Ulva Lactuca 
Harvey (1) 60. Robinson (1) 166. Hervey (1) 54.  Exsicc. 
Phyc Bor.-Am., 15. 
Var. VaHLII (J. Ag.) Rosenvinge (1) 949; M. VaA/ii J. G. Agardh, 
Till Alg. Syst., part 3, 109, Pl. III, figs. 84-89, 1882 ; De Toni, 
Syll. Alg., Vol. I, 106, 1889. Slenderer in form, often cylindrical, 
retaining its saccate shape longer, and with cells arranged in more 
or less distinct longitudinal series. Otherwise like the type. It 
occurs in Greenland and in Alaska, and has been found in the Mystic 
River marshes at Medford, Mass. At the Medford locality it was 
already formed when the ice broke up in the spring, and quite disap- 
peared in April. It occurred in a ditch where the water remained 
with little change of level from tides; the conditions were changed 
by *improvements" several years ago. since which time the plant 
has not been seen. Rosenvinge (1) 949; (2) 151. M. Vahli 
Dame & Collins (1) 157. Collins (4) 346; (8) 44. Saunders (1) 
410. 
3. M. arcTicum Wittr. Monogr., 44, Pl. II, fig. 8, 1866; J. G, 
Agardh, Till Alg. Syst, part 3. 106, 1882; De Toni, Syll. Alg., 
Vol. I, 105, 1889. 
Frond attached, at first saccate, soon splitting into a few broad 
laciniae; subradiately plicate, with crisped margin ; pale green, becom- 
ing yellowish in drying ; membrane 25-45 p thick; cells 4-6 angled, 
closely set, irregularly placed; in cross section either horizontally 
or vertically oval, 10-30 u high. Plate 41, fig. 6, cross section: fig. 
7, surface. 
A northern species, chiefly distinguished from M. Grevillei by the 
thicker frond, which is saccate only in the earliest stages, and after- 
wards appears as a rather broad membrane, not split up into narrow 
strips as is usual in M. Grevi//eí. Rosenvinge, who was familiar 
with it in Greenland, unites with this species three others, M. 
saccodeum Kjellm.; M. cylindraceum Kjellm.; and M. angicavum 
Kjellm.; and then reduces the whole to var. arctica of M. Greviliet. 
He says that it is impossible to draw the line between the forms 
with a thickness of 25 p and those reaching 60,4; or between those 
with cells in cross section horizontally oval and those vertically oval. 
In forms so closely related and passing into each other so much as 
do the forms of Monostroma, it is more a matter of personal pref- 
