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., Chaetbmorpha aerea, Entcromorpha intestinalis, 

 Cladophora sericea, ft rupestris, and JJlva 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 vireseens. 



Leathesia difforinis. 



Chorda filum. 



Cystoseira granulata. 



Halidrys siliquosa. 



Dictyota dichot. var. intricata. 



Porphyra limb, laciniata. 

 Helminthora divaricata (on Poly ides). 

 Gelidium spp. 



Chondrus crispus (broad form). 

 Gymnogongrus norvegicus (r). 

 Chyloeladia kaliformis. 

 Laurencia pinnatifida. 

 L. hybrida. 



Polysiphonia nigrescens. 

 Spermothamnion Turneri (epiphytic). 

 Dumontia filiformis. 

 Lithophyllum incrustans. 



(b) Sub-littoral Region. 



Ehodochorton floridulum Association. 



As before mentioned, this psammophilous 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 Lithothamnium 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, 



