16 58 Proceedings of the Royal Irish Academy. 



attached to low-lying rocks, and are thus not only exposed to constant sand- 

 movement, but are liable to be completely buried. On the other hand, with 

 certain winds, nearly all the sand is removed by the tide; and the thalli lie on 

 the bare rock. They are also liable to be covered by blown sand. Under 

 average conditions it would appear that only a few inches at the base of the 

 fronds are buried, and that the upper part remains free, lying flat oii the damp 

 ground : though one must assume that germination took place on rock. 

 Associated with Porphyra are Enteromorpha spp., especially the large flat 

 forms approaching E. lima. This plant oceurs in the damper positions, being 

 most abundant in the lower part of the association, and reaching down to the 

 sublittoral region. In August the Enteromorpha growth was in all cases 

 much less than in May. 



The zone formed by Porphyra on sheltered coasts is more extensive than 

 in the open. In the sandy bays described above the plant escapes a certain 

 amount of desiccation, and the result of this is seen in the fact that whilst on 

 bare rocks it does not exceed the F. spiralis level (at all events during summer), 

 on sand -covered rocks it frequently reaches Pelvetia. The lower limit also 

 is further than that of the exposed association, for the plant is frequent at 

 low-water mark, and occasionally extends well into the sub-littoral region. 



The form assumed in sheltered localities is that of the divided and lanceo- 

 late type, var. laciniata J. Ag. This is specially well developed on damp 

 rocks and on margins of pools. On drier rocks and with increased exposure 

 there is a tendency for the fronds to become short and umbilicate. The sand- 

 rock form of the association is well seen at Mulranny, also between Old Head 

 and Leckanvy, whilst a small patch exists in the bay on Clare Island. 

 Though not noted, forma linearis probably exists in winter in the upper 

 levels, as in exposed localities. 



A similar vegetation of Porphyra would, judging by the localities described 

 by Eosenvinge, appear to be widespread in Denmark ('09, p. 65). 



Laurencia-Gigartina Association. 



Fairly well developed in many parts of Clew Bay outside the islands. See 

 p. 33. 



Corallina-Lithothamnium Association. 



At times feebly developed in the more open parts of Clew Bay. See 

 p. 39. 



Corallina-Cladostephus Community. 



In company with the Laurencia community of sheltered coasts there often 

 occurs a similar vegetation of short dense growth, but composed largely of 



