From Rytiphlea complanata this species may always be known 
by its darker colour, cylindrical stems, and generally by a nar- 
rower frond. In ramification and general habit there is much 
similarity. The two may sometimes be found growing in close 
proximity, and even mixed together, but I have generally ob- 
served that &. thuyoides, which is the stiffest in substance, 
usually grows in the shallow parts of the tide-pool, sometimes 
standing out of the water; while &. complanata never dries 
during the recess of the tide. On the west coast of Ireland this 
is a very abundant plant, growmg on most rocky shores. It 
forms dense tufts of large size, but is often much stunted, and 
is only to be found well grown in the deeper pools near low-water 
mark. 
From &. fruticulosa the erect habit and more regularly pin- 
nate ramification distinguish it. In some specimens these cha- 
racters are less strikingly manifest than in others, but it rarely 
happens that the branching is so patent or irregular as to cause 
the specimens to be mistaken for one of the former species. 
Small specimens of Polysiphonia nigrescens much resemble the 
present species in habit, but are at once known under the micro- 
scope, by the very different structure of the frond. 
Fig. 1. Rytipaia#a THUYOIDES :—of the natural size. 2. Branch with Cera- 
midia. 3. Ceramidia. 4, 5. Branchlets from different specimens. 6. 
Portion of the stem. 7. Transverse section of the same :—all move or less 
magnified. 
