15 42 Proceeding* of the Royal Irish Academy. 



month, but from one year to another. A detailed study and analysis of the 

 vegetation was impossible; but a series of observations was made, and a 



general account is given below. 



In the upper pools an encrusting vegetation is always conspicuous. At the 

 highest levels HUdenbrandtia, prototype is found, especially on rugged rocks 



and on exposed dill's. Slightly below, and generally in llattei areas, Ral/sia 



verrucosa appears, covering the bottoni of shallow j Is with a thick crust. 



and often completely ousting HUdenbrandtia. At a lower level (probably 

 that of ordinary high-tide)., LUkophyllum inctustana begins t<> develop, tn- 

 a very thin crust which does not fruit; then rapidly increasing in vigour on 

 the lower rocks, it passes into the Lithothanmium association (p. 41). Ralfsia 

 and Lithophyllmn are, in the upper shallow pools, in competition. The 

 former cakes off with age, allowing Lithophyllum for a time to get the upper 

 hand. Ralfsia, however, tolerates more fresh water, and probably grows more 

 rapidly. Both can withstand intense insolation, and often form large sheets 

 without any protecting vegetation whatever. The water of these pooh 

 becomes in summer very warm. 



With regard to the upright vegetation, ErUeromorpha intestinalu is often 

 alone in the highest pools, but is joined by Cladophora sericea, Chaetomorpha 

 aerea, and Scytosiphon lomcntari/'s, where the salt-water is constantly renewed 

 by spray and splashing. Below these a series of fairly characteristic pools, 

 situated about high- water level, is reached, which are well supplied with 

 seaweeds during winter and spring. But in summer the larger algae are 

 scarce, or represented by a few hardy species, usually infested with Diatoms 

 and Cyanophyceae. The dominant species are : — 



Cladophora rupestris. Gigartina mamillosa 



C. sericea. Laurencia hybrida (except summer). 



Scytosiphon louientarius. Polysiphonia fruticulosa. 



Leathesia difformis. DumontianTiformis (except summer). 



Gelidium sp. Corallina officinalis. 



The additional winter and spring flora need not be listed. The pools are 

 not always replenished in calm weather, and are distinctly brackish ; they are 

 about 6 inches deep, and commonly run parallel with the shore. 



At a lower level the typical purely marine rock-pools commence, in varied 

 form, and with abundant vegetation. They are much more difficult to classify 

 and have a constantly changing flora. With a sufficient depth, large algae, 

 such as Halidrys, Cystoseira fibrosa, &c, are plentiful, and provide shade: hence 

 almost any of the shade-loving Florideae (except those which require daily 

 exposure to the air) may be met with. If shallow, the flora is more limited. 



