MARINE ALGAE FROM EASTER IS! AND 



253 



The flabclliform mode of ramification reminds very much of Anadyomene, 

 but in Rhipidiphyllon the space between the large cells remains open no small 

 cells being formed here. ASKENASY described the plant as an Anadyomene, 

 but I think HEYDRICH was right in making it the type of a new genus. 



Rhipidiphyllon differs from Microdictyon mainly by its acropetal, fanshaped 

 ramification resulting from the fact that the new cells are produced from the 

 upper end of the mothercell, all being directed upwards under acute angles 

 and all lying in the same plane. It also differs in the less regular and less 

 frequent junction of the cells. Rhipidiphyllon is also a much smaller plant; the 

 largest specimens I have seen are about 5 — 6 mm in diameter, as stated by 

 ASKENASY. 



It seems to me rather doubtful if the plant so beautifully figured by 

 OKAMURA in his »Illustrations of the Marine Alga; of Japan*, Vol. 1, no. 6, 

 pi. 30 really belongs to this genus. I am much more inclined to consider this 

 a Microdictyon. The shape of the thallus, the mode of ramification, the shorter 

 and less marked main cells, the more spreading branches, not distinctly gathered 

 at the end of the main cells with much larger angles between them, and the 

 lowermost being directed downwards, all suggests Microdictyon. 



Area of distribution: West Australia, Formosa, Easter Island. 



Microdictyon Dene. 



M. umbilicatum (Veil.) 

 Zanard. — Fig. 5. 



Zanardini, Iconographia, 

 Phycologica Adriatica, vol. I, 

 p. 79, tab. XIX, i860. 



Some few small frag- 

 ments were found growing 

 on Hypnea. 



Regarding the limita- 

 tion of species in Microdictyon 

 there still is some uncertainty, 

 as pointed out by REINBOLD. 1 

 In the treatise quoted below 

 he points out that the net- 

 work arises in three different 

 ways, namely by means of 

 a simple anastomosis (»Ver- 

 dickungsring», Bitter) as is 

 the case in M. umbilicatum, 

 by pseudohapters, the attach- 



] REi\ T BOLD,TH.,in A.Weber 

 van Bosse, Liste Algues Siboga, 

 I, p. 66. 



Fig. 5. Microdictyon umbilicatum (Veil.) Zanard., part of 

 specimen, c. 85 / 1 - 



