15 62 Proceedings of the Royal Irish Academy. 



psammophilous vegetation is found, of which Rhodochorton is a good example. 

 With extreme shelter, this can form a mossy carpet on pure sand ; but as a 

 rule it is only found on sand-covered rocks, or on small intervening sand- 

 spaces. The alga is probably in the first instance attached to rock, but as the 

 plants grow they form adventitious rhizoids and accumulate sand. Being 

 saxicolous, Rhodochorton extends to rocky shores, where sand is scarce 

 or even absent. As an association, however, it is only found where this 

 material is plentiful. Several other psammophilous species accompany 

 Rhodochorton. 



The Corallina-Cladostephns community represents the last stage in the 

 transition from sand to rock. Though frequent on sandy shores, a bed of 

 rock is essential for its growth, hence it has been dealt with under the rocky- 

 shore formation. The sand-pool vegetation is, on the other hand, described 

 in the present section, as the characteristic species, though attached to rock, 

 are markedly psammophilous. The sub-littoral associations call for no special 

 comment, except that the last three are not strictly psammophilous, though 

 in our district they are confined to sandy areas. They are placed under this 

 beading for convenience, and therefore provisionally. 



The associations and communities recognized in the sand and sandy-mud 

 formation are as follows : — 



(a) Littoral Region. 

 Sand and sandy-mud series 



Sand and low-rock series : 



Rhizoclonium riparium. 

 Microcoleus chthonoplastes. 

 Vaucheria Thuretii. 

 Vegetation of vertical banks. 



Rhodochorton noridulum. 

 Corallina-Cladostephus. 

 Porphyra umbilicalis. 

 Sand-pool vegetation. 



(b) Suh-littorcd Region. 



Rhodochorton floridulum. 

 Zostera marina. 

 Fruticulose Lithothamnium. 

 Pebble-attached association. 

 Encrusting algae. 

 Channel-vegetation. 



