1903] Collins, — The Ulvaceae of North America II 
one species lives in fresh water exclusively, and some of the marine 
and brackish species occasionally occur also in fresh water. 
About 30 species have been described, but the differences between 
some of them seem to be largely due to the age of the specimen or to 
local conditions; the following arrangement is based chiefly on 
Rosenvinge's work on the algae of Greenland, where this genus is 
given a careful study, based on an ample supply of living plants. 
KEv TO THE SPECIES. 
1. Frond always tubular 2 
Le * tubular only in early stage of growth 3 
2. 'Tube filiform 10. M. GROENLANDICUM 
2. * jintestine-like, collapsing 3a. M. ARTICUM var. INTESTINIFORME 
3. Frond in the full-grown plant with a long, slender stipe; membrane not 
over IO p thick 6. M. LEPTODERUM 
3. Frond never distinctly stipitate 4 
4. Frond saccate for a considerable part of the growth of the plant; 
then splitting part of or all the way to the base 2. M; GREVILLEI 
4. Frond saccate only in the very early stages or not at all 5 
5. Frond dark or dull green, not gelatinous nor adhering to paper. 
1. M. FUSCUM 
. Frond light or bright green, more or less gelatinous, adhering to paper 6 
wn 
6. Mature trond div ided into distinct segments 7 
6. Frond from broadly lanceolate to orbicular; not divided into seg- 
ments 
7. Segments linear or lanceolate ; frond about 6 p thick above 
4. M. PULCHRUM 
few, broad, of no fixed form ; frond 25-45 p thick; cells closely 
set 3. M. ARCTICUM 
7. Segments obovate, frond 18-36 p thick; cells not closely set 
7. M. CREPIDINUM 
[11 
-I 
8. Frond not over 30 p thick 9 
8. “ 40-50 p thick 5. M. UNDULATUM 
9. Frond usually not much plicate 10 
“ * much plicate 5a. M. UNDULATUM var. FARLOWII 
10. Cells arranged in distinct groups of 4 9. M. QUATERNARIUM 
10. ‘“ without order, or indistinctly in groups of 2, 3, or 4 
. M. LATISSIMUM 
1. M. ruscuM (Post. & Rupr.) Wittr., Monogr., 53, Pl. IV, fig. 13, 
1866; J. G. Agardh, Till Alg. Syst., part 3, 113, 1882 ; DeToni, Syll. 
Alg., Vol. I, 109, 1889. 
Frond membranaceous, at first tubular, soon splitting, dull or dark 
green, more or less lobed but not divided to the base; membrane 
20-70 p thick; cells 4-6 angled, very closely set; in cross section 
from square to vertically elongate, with only slightly rounded corners; 
occupying nearly the entire thickness of the membrane. 
Under this species are included three which were formerly, and by 
some are still kept distinct; M. fuscum with frond of dull color, 
20-35 p thick and cells quadrate in cross section; M. BZyttii with 
deep green frond, blackish in drying, 60-70 y thick and cells “ pal- 
