15 36 Proceedings of the Royal Irish Academy. 



Cattithamnion arhuscula was more abundant. Apart from these seasonal 

 variations, Ceramium is found chiefly on bare slopes with much or moderate 

 exposure; sometimes it occurs pure, forming a dense carpet several yards 

 square, but more often it is mixed and interrupted. Callithamnion prefers 

 the more boisterous positions, and is especially luxuriant on steep or vertical 

 rocks. 



The belt formed by the association is at the same level as the lower part 

 of the Bangia and Porphyra associations, or slightly higher than the Nemalion 

 and Corallina communities. It usually begins just below F. spiralis, and extends 

 to within a few feet of low-water mark. Calhthamnion often appears a 

 short distance above Ceramium, but for the most part the two species occupy 

 the same level. Where fresh- water streams exist, Ceramium not infrequently 

 ascends the shore, and may reach as high as Pelvetia. G. arhuscula, on the 

 other hand, does not thrive in fresh water. 



Unlike the Bangia-TJrospora association, the present plant-community is 

 most frequent on rough surfaces offering a good foothold. Ceramium, which 

 forms a spreading growth, usually occurs on mussels and Corallina, amongst 

 which it pushes a mass of rhizoids ; but it also grows epiphytically on the 

 Callithamnion. The latter, on the contrary, has a scattered habit, each 

 plant possessing a single stem, which is thick and spongy. It is most 

 frequently attached to limpets, mussels, or the rock between them ; only 

 rarely is it found on other algae. Although both species occupy a fairly 

 high level, owing to the retention of water by their spongy thalli neither 

 becomes dry when the tide is out. 



The associations into which the Callithamnion arhuscula vegetation most 

 frequently merges are those of Bangia, Nemalion, Corallina ; and in addition 

 to algae derived from these sources the following species frequently occur 

 scattered in it : — 



Rhizoclonium tortuosum. Polysiphonia niacrocarpa. 



Enteromorpha compressa. P. Brodiaei. 



Scytosiphon lomentarius. P. thuyoides. 

 Petrospongium Berkeley i. 



Comparison with other Localities. — In the British Isles Borgesen has 

 noted the community in Shetland, and I have observed it at Whitby 

 (Yorks). On the south coast of England, where C. arhuscula is absent, the 

 association is represented by a belt composed entirely of C. granulatum 

 (= C. spongiosum, Harv.). This has been noted at Swanage and Portland, and is 

 doubtless common. Ceramium acanthonotum, though known on the south coast, 

 does not enter into the Callithamnion association in either of these localities. 



