313 
British Marine Algae. 
branches ; plurilocular sporangia broadly conical or ovoid, 
surrounding the axis with dense cushion-like masses. M. 
filiformis , Harv. 
I propose to characterize M. densa as follows : — 
Fronds dark olive-brown, almost black, cylindrical in 
outline, slightly attenuated at the base, from half an inch to 
an inch and a half in length and from 200-250 ju, in diameter, 
axis densely clothed throughout with short dichotomous or 
secund level-topped more or less moniliform secondary 
branches, hyaline hairs few or altogether wanting ; unilocular 
sporangia stalked, spherical or ovoid, about 50 \x in diameter ; 
plurilocular sporangia cylindrical or lanceolate, 25-60 ju, long 
and 6-10 [a in diameter, containing one or two rows of cells. 
Thickly fringing the old fronds of Zoster a marina. Wey- 
mouth, Cumbrae, Arran, E. A. B. ; Swanage, T. H. Buffham. 
Tellamia contorta, Batt. 1 . c. 
At almost any station along our coasts shells of the com- 
mon yellow periwinkle ( Littorina obtusata , L.) may be found 
which have assumed a more or less deep olive-green colour. 
On examination with the microscope it will be found that 
this discoloration is caused by the presence of filiform Algae 
which penetrate the periostracum of the shell, but so far as 
I have been able to ascertain do not enter the calcareous 
portion of it. Although these Algae are usually found in 
abundance in shells inhabited by the living molluscs, where 
of course the periostracum is more or less perfect, I have not 
found them in dead shells, where it has been entirely worn 
away, so that I feel the more confident that these Algae 
confine their attacks to the periostracum, although they 
usually make their appearance, and no doubt find an entrance, 
where that membrane has been more or less eroded at the 
umbo. The presence of the Algae seems in no way to injure 
the shell, and may be of direct advantage to the mollusc, as 
the affected shells are much less conspicuous amongst the 
dark olive Fuci on which they are usually found than the 
bright yellow and consequently very noticeable healthy ones. 
