15 62 Proceedings oj the Royal Irish Academy. 



psammophiloua vegetation Lb found, of which Rhodoehorton is a g 1 example. 



With extreme shelter, this can form a mossy carpel on pure Band; but as a 

 rule it is only found on Band-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 

 3axicolous, Rhodoehorton extends to rocky Bhores, where Band is scarce 

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

 materia] is plentiful. Several other psammophilous species accompany 

 Rhodoehorton. 



. The Coralliua-Cladostephus 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 

 heading for convenience, and therefore provisionally. 



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

 formation are as follows : — 



(a) Littoral Region. 

 Sand and sandy-mud series 



Sand and low-rock series 



Ehizoclonium riparium. 

 Microcoleus chthonoplastes. 

 Vaucheria Thuretii. 

 Vegetation of vertical banks. 



Rhodoehorton floridulum. 

 Corallina-CTadostephus. 

 Porphyra umbilicalis. 

 Sand-pool vegetation. 



(b) Sub-littoral Region. 



Rhodoehorton floridulum. 

 Zostera marina. 

 Fruticulose Lithothamnium. 

 Pebble-attached association. 

 Encrusting algae. 

 Channel- vegetation . 



