Marine Algae from Eniwetok — Dawson 
99 
72. Growing point without a distinguish- 
able apical cell Grateloupia 
73. Thalli small, narrow, less than 1 cm. 
tall; medulla of several layers of small 
cells 74 
73. Thalli moderately large, 3-10 cm. tall; 
medulla of a single layer of large cells . . 
Dictyota (in part) 
74. Medulla with rhizoidal filaments run- 
ning through it Gelidium 
74. Medulla without rhizoidal filaments 
running through it 
Gelidiella (bornetii) 
75. Axes and branches all similar in ap- 
pearance and structure 80 
75. Axes distinct from ultimate branchlets 
in appearance and structure 76 
76. Branchlets produced in whorls 
Wrangelia (in part) 
76. Branchlets produced alternately or ir- 
regularly 77 
77. Branchlets not banded 78 
77. Branchlets banded Spyridia 
78. Branchlets multiseriate; plants rather 
large, 6-12 cm. tall Asparagopsis 
78. Branchlets uniseriate; plants small, usu- 
ally less than 2 cm. tall 79 
79. Main axes completely or incompletely 
corticated Dasya 
79- Main axes uncorticated 
Heterosiphonia 
80. Thallus parts of essentially uniform cell 
structure throughout; not banded al- 
though sometimes appearing segmented 
81 
80. Thallus parts provided with discontin- 
uous cortication in the form of bands 
around a large axial cell row, the bands 
usually close together above and well 
separated below Ceramium 
81. Thalli of various reddish and brownish 
colors, but not grass green .82 
81. Thalli grass green in color, entangled. . 
Enteromorpha (ralfsii) 
82. Branches provided with whorls of short, 
tooth-like spines, at least in part 
Centroceras (in part) 
82. Branches without whorls of spines. .83 
83. Middle and upper thallus parts showing 
only a few cells (2-5) across the diam- 
eter when seen in surface view 84 
83. All thallus parts showing many (10 or 
more) small cells across the diameter 
when seen in surface view 89 
84. Branches termination in a large apical 
cell of nearly the same diameter as the 
filament bearing it Sphacelaria 
84. Branches terminating in an apical cell 
which is much smaller than the diam- 
eter of the filament bearing it 85 
85. Thalli with definite axes provided 
with dense, multifarious, divaricately 
branched short branchlets which in 
mature plants become attached to each 
other Tolypiocladia 
85. Branching not as above 86 
86. All branches essentially of unlimited 
growth 87 
86. Branches of potentially unlimited growth 
alternating with groups of branches of 
definitely limited growth 
Herposiphonia 
87. Erect filaments arising from extensive 
prostrate creeping filaments 
Lophosiphonia 
87. Without extensive prostrate filaments. 
88 
88. Mature filaments composed of cells in 
tiers of three Falkenbergia 
88. Mature filaments composed of cells in 
tiers of four or more. . . . Polysiphonia 
89. Surface cells not arranged in regular 
rows, nor the thallus appearing seg- 
mented 90 
89- Surface cells arranged in horizontal and 
vertical rows and the thallus appearing 
somewhat segmented 
Centroceras (apiculatum) 
90. Thallus without a central core of small 
cells in the medulla 91 
90. Thallus in cross section showing a core 
of very small cells in the center of the 
medulla Dicranema 
