Clare Island Survey — Marine Atyae. 15 31 



to that of Porphyra, extending from below the Pelvetia zone to within a 

 few feet of low water. Its composition is as follows : — 



Bangia fuscopurpurea, \ 



Ulothrix pseudoflacca, > co-dominant. 



Urospora mirabilis, ) 



Ulothrix consociata, 



Enteromorpha minima. 



As a rule all the species, except E. minima, which is local, occur together, 

 though each forms a separate patch or narrow strip within the association. 

 Occasionally, however, the species occur separately; and a pure growth of 

 Bangia or Urospora may cover a wide area. In a few cases the growth may 

 be thoroughly mixed. 



An effort was made, when the full series was present, to determine 

 the vertical sequence of the species ; but this was unsuccessful in so far as 

 obtaining a constancy in succession was concerned. The necessity of 

 microscopic examination made the determination in the field difficult, and 

 the work required more time than was available. As a general rule, however, 

 where the association is typically developed, Bangia appears to have a wider 

 vertical distribution than the other species, as it is often both the first to 

 appear and last to leave on passing from top to bottom of the area. But 

 occasionally a Ulothrix growth is found at a very high level in sheltered 

 localities where Bangia is absent. 



The occurrence of the Bangia-Urospora association on smooth rocks and 

 boulders where other vegetation is absent is explained by the exceedingly 

 fine unbranched filaments of the plants, reducing to a minimum the strain 

 on the basal attachment. At the same time each filament is separately 

 fixed to the rock, so that the association may be said to be attached along its 

 whole base. 



The most usual extraneous species is Porphyra umbilicalis, small specimens 

 of which frequently occur in the upper levels ; and it is into the Porphyra 

 association that the present community most frequently merges. Bangia, 

 which is less restricted than Ulothrix or Urospora, also occurs on barnacles, 

 where it enters into competition with Ceramium acanthonotum. 



The total absence of Bangia in some years has been referred to by 

 Borgesen for Norway ('05, p. 720), and Rosenvinge for Denmark ('09). The 

 latter, who has studied the Bangiales of Denmark in detail, writes as follows 

 concerning B. fuscopurpurea : — 



" The most dangerous condition for the Bangia vegetation is a fairly long 

 period of easterly winds so light that this vegetation is not reached by the 



