Marine Plants of Nha Trang — DAWSON 
present, these not in whorls 
Galaxaura 
16. Thallus crustose 17 
Thallus not crustose 18 
17. Tetrasporangia in sunken conceptacles 
Hildenbrandia 
Tetrasporangia in nemathecial sori 
Peysonnelia (in part) 
18. Free thallus parts consisting of a single 
row of cells 19 
Free thallus parts consisting of more than 
a single row of cells 27 
19. Filaments unbranched 
Erythrotrichia 
Filaments branched 20 
20. Cell cavity less than half the diameter of 
the gelatinous filament . . . Asterocystis 
Cell cavity much greater in diameter than 
wall thickness 21 
21. Cells of subultimate segments over 160 
IJL in diameter Griffithsia 
Cells of subultimate segments less than 
100 in diameter 22 
22. Branching opposite or verticillate, at 
least below 23 
Branching alternate or irregular 24 
23. Tetrasporangia surrounded by curved, 
involucral filaments Wrangelia 
Tetrasporangia not associated with curved 
filaments Antithamnion 
24. Cells of main axes over 50 /x in diameter 
25 
Cells of main axes less than 20 /x in 
diameter. Acrochaetium 
25. Branches spirally arranged from a per- 
current axis 26 
409 
Branches sparse, not spirally arranged . . 
Neomonospora 
26. Sporangia to 150 /x in diameter, undivided, 
encircled by slender involucral filaments 
Callithamnion? 
Tetrasporangia to 50 /x in diameter, free 
Mesothamnion 
27. Thallus complanate and membranous, at 
least in part 28 
Thallus cylindrical, compressed or flat- 
tened but not membranous 32 
28. Thallus membranous throughout. . . .29 
Thallus consisting of a subcylindrical, 
prostrate axis bearing membranous, leaf- 
like blades Leveillea 
29. Growing by means of an apical cell. . . 30 
Growing by means of a marginal meris- 
tem 31 
30. Branches arising from the midrib 
Hypoglossum 
Branching subdichotomous . Caloglossa 
31. Thallus monostromatic Porphyra 
Thallus consisting of several layers of 
cells and filaments Halymenia 
32. Thallus uniaxial, a single apical cell dis- 
tinguishable at least in young, growing 
parts 33 
Growth by means of a marginal, terminal, 
or intercalary meristematic region. ... 55 
33. Thallus furry, consisting of a densely 
corticated axis bearing abundant, uni- 
seriate hairs 34 
Not as above 35 
34. Branching dichotomous .... Dasyopsis 
Branching multifarious Dasya 
35. A central axial filament readily observed 
