Algae. 
69 
the fertile States of H. Tuna, from Culebra Island, off Porto 
Rico, Howe 4201, and of H. tridens, from near Tallaboa, on 
the Southern shore of Porto Rico, Howe 4424, comparing them 
in detail with each other, particularly as to the histology of their 
reproductive parts. 
B. On the American species of the Halimeda Tuna 
group. Halimeda Tuna is known from American waters on specimens 
from Bermuda, Porto Rico, and Jamaica (also from Florida?), 
the specimens showing relatively slight differences from those of the 
Old World. Two closely allied species occur in American waters: 
H. scabra, Howe (1905) and a “smooth plant of the Tuna-cuneata 
alliance” regarded by the author in 1905 as a species inquirenda. 
This last is now definitely placed as H. discoidea Decaisne (1842) and 
is fully described and figured. Recently it has been confused mostly 
with H. Tuna forma platydisca (Decaisne) Barton, which is simply a 
very large condition of H. Tuna. 
C. On the American species of the Halimeda tridens 
group. A critical microscopic examination of 57 separate numbers 
of the two “forms” reveals ample and trustworthy histological 
characters distinguishing Halimeda tridens and H. Monde , species 
which occasionally simulate each other in form. In H. tridens the 
peripheral utricles show a ränge of from 49 to 77 ju in average 
maximum diameter in surface view; in H. Monile from 30 tot 44 fx. 
Other distinctive histological characters are mentioned. — Another 
species allied to H. tridens, H. Monile, and H. favulosa, is described 
as H. simulans Howe, sp. nov., the type being from Porto Rico, 
Howe 4332. This has been collected by the writer also in Jamaica 
and the Bahama Islands. It is tigured and compared with its 3 
allies, and a key to all 4 species is published. 
D. On the sporangia of Avrainvillea nigricans. A fer¬ 
tile specimen of Avrainvillea nigricans, collected by the writer at 
Mont eg o Bay, Jamaica, January 5, 1907, furnishes the basis of 
an account of the method of reproduction in a member of this genus, 
concerning which nothing had previously been known. The sporangia, 
terminal on the branches, are described as to form and dimensions, 
and practically all stages of development are described and figured. 
Sometimes practically the whole protoplast of a sporangium is con- 
sumed in the formation of a single Spore; but the usual number of 
spores is 3, 4, or 5, though occasionally 6, 7, or even 8 are formed. 
These are commonly long-ovoid or pyriform, the broader end usually 
lying upward in the sporangium. “They are densely crowded with 
chloroplasts and starch-grains, but as they mature the amount of 
starch appears to increase at the expense of the chlorophyl.” Some 
show a pronounced polarity, or perhaps it may be the beginning of 
germination, while still in the sporangium. The membrane enclosing 
the spore is usually thin and delicate and often scarcely demon- 
strable. “The large size of the spores (130—300 u X 66—120 ,u), their 
heavy load of starch, and their occasional development of an evident 
foot while still in the sporangium”, are held by the author to make 
it practically certain that these bodies are not self-motile, i. e., that 
they are aplanospores. “Yet, the thinness'of the surrounding 'mem¬ 
brane and the fact that none of the bodies has thus far been found 
germinating on the surface of the thallus.have suggested the 
possibility of their being synzoöspores.” 
E. On the American species of Avrainvillea. The 
genus Avrainvillea is treated synoptically as to the American 
