15 68 Proceedings of the Royal Irish Academy. 



Gymnogongrus and Gelidium were, however, not found outside sand-pools. 

 Where the sides are steep, ordinary saxicolous species naturally occur; 

 but as a rule the sides are shelving and the pools shallow. When fresh 

 water exudes between the rocks, the Ehodophyceae are replaced by 

 Chlorophyceae — i.e., Chadomorpha aerca, Enteromorpha intestinalis, 

 Cladophora sericea, C. rupestris, and Ulva Lactuca ; Gracilaria, however, 

 usually remains, being able to withstand a considerable quantity of fresh 

 water. 



Where the sand-layer is very thin a number of other species are found, 

 and being characteristic of sandy shores, the most usual amongst them are 

 listed below : — 



Ulva Lactuca (very thick form). 



Enteromorpha Linza. 



Monostroma Grevillei (epiphytic). 



Scytosiphon lomentarius. 



Punctaria latifolia. 



Chordaria flagelliformis. 



Eudesme virescens. 



Leathesia difformis. 



Chorda filum. 



Cystoseira granulata. 



Halidrys silicpiosa. 



Dictyota dichot. var. intricata. 



Porphyra umb. laciniata. 

 Helminthora divaricata(on Polyides). 

 Gelidium spp. 



Chondrus crispus (broad form). 

 Gymnogongrus norvegicus (r). 

 Chylocladia kaliformis. 

 Laurencia pinnatifida. 

 L. hybrida. 



Polysiphonia nigrescens. 

 Spermothamnion Turneri (epiphytic). 

 Dumontia filiformis. 

 Lithophyllum incrustans. 



(b) Sub-littoral Region. 



PJiodochorton floridulum Association. 



As before mentioned, this psammophiloiis species descends into the 

 sub-littoral region, covering large areas where the bottom is flat and very 

 sheltered, as in the inner part of Clew Bay. The association apparently 

 exists in certain spots as an unmixed growth ; but as a rule it is more or less 

 interspersed amongst Lithothamniuni or other algae (see p. 65). 



Zostera Association. 



The soft bottom of the inner part of Clew Bay is admirably suited to the 

 growth of Zostera, and in the muddy parts especially, extensive beds are 

 found. The characteristic algal epiphytes are also well-developed, being most 

 marked in spring and early summer. 



Ostenfeld in his report on Zostera ('08) points out that, as a general rule, 



