This is the coarsest of the British Callithamnia, resembling, 
in its bushy habit, shaggy tufts of Sphacelaria scoparia, and is 
often of a very dark brown colour, with little trace of the purple 
endochrome characteristic of the genus. At other times, much 
more shaggy specimens with a brighter colour are found, and 
some of these resemble large specimens of C. Borreri. Speci- 
mens collected at various seasons and from different localities 
differ much in the abundance and regularity of the plumules, as 
well as in the greater or less development of the hair-like ramuli. 
‘There are always, however, characters to be found quite sufficient 
to separate C. ¢etricum from any British species. 
Mr. Dillwyn was the first author to notice this species, 
having found it abundantly in the neighbourhood of Swansea. 
He states that it is found generally in pools left by the tide, 
attached either to rocks or parasitical on Alge. I have more 
commonly found it on the perpendicular faces of rocks which 
are left bare at low water, either perfectly exposed or shaded 
by hanging fronds of Fucus serratus and nodosus. In such 
situations, on the west of Ireland, it may often be found covering 
large spaces of rock, and growing luxuriantly. 
None of our British species are very closely related to C. 
tetricum, but it is interesting to find in the C. hirtum of Auck- 
land Islands, and C. scoparivm of the Falklands, its southern 
analogues, resembling it in many respects, but readily distin- 
guishable by obvious characters. 
Fig. 1. CALLITHAMNION TETRICUM, tufts :—of the natural size. 2. Portion of 
an upper branch :—slightly magnified. 3. A plumule with tetraspores. 4. 
Ramulus from the same. 5. A plumule with favellae:—more highly mag- 
nified. 
