British Marine Algae . 319 
and plate of Callithamnion serpens given by the Crouans leads 
me to the belief that the plant found at Plymouth belongs to 
the same species. There can be no doubt, however, that 
the plant does not belong to the genus Callithamnion , and 
I propose to make a new genus Hymenoclonium for its 
reception. 
This pretty little species creeps over the surface of the 
stone, or whatever it grows on, and adheres by its entire 
under surface, there being no rhizoids or anything of the sort. 
It is of a beautiful pink colour ; and being branched like 
a Callithamnion or Ptilota , with the branches united by 
a hyaline membrane, it looks like a pink fern in miniature. 
The branches are all opposite, and either anastomose or fit 
between each other so as to form a nearly parenchymatous 
expansion, which retains its fern-like outline, however. There 
are no erect branches. Sometimes the main stem extends 
beyond the others and is bare of branches. According to the 
Crouans, tetraspores are formed from the contents of some of 
the cells of the main branches, but these I have not seen. 
The new genus may be described thus — 
Hymenoclonium, nov. gen. 
Fronds minute, adhering by the entire under surface ; 
branches opposite, frequently anastomosing, united by a hya- 
line membrane into a pseudo-parenchymatous expansion ; 
tetraspores (according to the Crouans) cruciate, formed from 
the cells of the main stem. 
Hymenoclonium serpens (Crn.) = Callithamnion serpens , Crn. 
Ann. Sc. Nat., fourth ser., vol. xii, p. 296, pi. 22, 1 , Figs. 41-43 ; 
blonde du Finist&re, p. 135. 
Fronds minute, 2-4 mm. in length, rose red or pink, adher- 
ing to the substratum by their whole under-surface, bi-pinnate, 
branches opposite united by a hyaline membrane into an 
irregular pseudo-parenchymatous expansion, upper portion 
of the main branches often naked ; cells of the main stems 
35-40 ix long and 10-12 /ut in diameter, those of the 
secondary branches 10-20 jx long, 8-10 [x or more wide ; 
