15 ti Proceedings of the Royal trish Academy. 



(b) In summer Striaria, Dictyosiphun, Delesseria, and Griffithsia largely 



disappear, but the other species remain ; and in addition we find : — 



Cladophora rectangularis. Polysiphonia subulifera. 



Spermatochnus paradoxus. P. violacea. 



Castagnea virescens (shallow 1'. fibrillosa. 



water). Brongniartella byssoides. 



Dictyota dichotoma. Ceramium spp. 



As autumn advances one species after another becomes detached and 

 disappears, though Dictyosiphon, Ectocaipus, and Polysiphonia nigrescens 

 remain. A day's dredging in February showed that most of the spring species 

 had started growth ; but the vegetation on the whole was very scanty com- 

 pared with that of spring and summer. It should be noted that a few of the 

 species above listed may be largely epiphytic on the others, and that in summer 

 the upper part of the association is very poor. 



Very many other algae, both littoral and sub-littoral, enter into this 

 vegetation. Several of these are characteristic of other associations, 

 e.g. Gracilaria, Polyides, and Ehodoclwrton floridulum. (Jallithamnia likewise 

 are very plentiful, and the encrusting alga association is often found as an 

 undergrowth. 



Encrusting Alga Association (sheltered). 



In spite of unwillingness to change names, a new term is introduced here. 

 The Lithoderma formation described by Kjellmann has been recorded by 

 other observers ; but the difficulty of determining encrusting species makes it 

 often impossible to be certain if Lithoderma is really dominant, or whether 

 other algae of similar appearance are not equally common. With regard 

 to Clew Bay, it is not certain that Lithoderma is even present ; though, 

 judging from sterile specimens, it is probably abundant. A wider and more 

 general term has therefore been selected. 



The association is found on a soft bottom overlaid with shells and fiat 

 stones, which, instead of supporting a bushy vegetation like the last-named, 

 are practically covered with encrusting algae. Of these, the first four in the 

 following list are conspicuous and easy to recognize; the remainder are 

 probably common, though not so readily detected : — 



Ralfsia verrucosa. Petroderma maculiforme, /'. 



Aglaoiconia reptans. Lithothamnium hapalidioides, J. 



Cruoriella Dubyi. L. Lenormandi, car. sublaevis, /. 



Lithophyllum incrustans. Melobesia zonalis, r. 



Ralfsia clavata,/. Peyssonnelia sp., r. 

 Lithoderma fastiscens ?,/. 



