15 64 Proceedings of the Royal Irish Academy. 



de sel sans melange cle vase. En cet ^tat il forme des assises diversement 

 color^es, accumul^es sur une epaisseur parfois considerable et dont les 

 superieures, qui sont les seules vivantes, peuvent etre detachees par larges 

 plaques et routes eomme des morceaux de drap. Ce vegetal nitrite 

 done completement le nom de chthonoplastes (constructeur du sol) qui 

 lui a 6t6 donne par les premiers observateurs." 



In Clew Bay the Microcoleus association was usually found where there 

 was a certain amount of mud. It occurs in large patches at Mulranny and 

 also in the Belclare neighbourhood. In the small bay to the north-west of 

 Achill Bridge it forms a broad band just below high-water line, binding the 

 upper layer of mud into, a firm sheet. It was also noted in several localities 

 in fairly pure sand, but only as covering small areas. 



Various other blue-green algae enter into the composition of the 

 association, notably : — 



1 n upper parts : In lower parts : 



Lyngbya aestuarii. Yaucheria Thuretii. 



L. lutea. Bhizoclonium riparium. 



Microcoleus tenuis. Enteromorpha torta. 



Vaueheria Thuretii Association. 



This vegetation takes us further still from the pure sand, being found only 

 where there is a considerable proportion of mud. Future work will probably 

 show that it should be regarded as an association in the mud-formation. 

 V. Thwetii forms a dense mossy growth embedded in the matrix, and appears 

 to cover very extensive areas. Several samples were taken both from Clew 

 Bay and from the muddy inlets north of the bridge at Achill Sound; and in 

 each case the mossy tuft consisted of this species, often in beautiful fruiting 

 condition, and practically pure. The growth is always immersed, the tips of 

 the filaments alone showing, thus differing markedly from V. coronata, which 

 forms a clean velvety growth. Felts composed of Bhizoclonium, Chaetomorpha 

 and Cladophora fracta often lie on top of the Vaueheria, and these, in many 

 cases, appear to be attached, and not merely drifted fragments ; so that it is 

 possible that careful .study would reveal definite societies of other species 

 amongst the general vegetation of Vaueheria. 



The vertical range of V. Thuretii is determined by the extent of the sandy 

 mud. It flourishes in the lower part of the littoral region, and probably 

 descends to low-water mark. Its upper limit is usually immediately below 

 F. spiralis, but whether this was due to the lack of sufficient mud or to other 

 causes in addition (such as long exposure to the air) was not ascertained, On 



