Clare Island Survey — Marine Algae. 16 41 



with a widespread growth of that alga. A thorough study of its occurrence 

 in varying conditions was not made ; consequently the following notes must be 

 regarded as but a contribution to the study. 



The growth in question occurs in three zones, Lithothamnium Lcnormcmdi, 

 the uppermost, IAthophyllum incrustans, the second, and the third, 

 Lithothamnium polymorphum, appears only when low-water mark is 

 reached. All three plants resent desiccation, and cannot bear being dried 

 up. L. polymorphum, and, to a lesser degree, L. Lenormandi, prefer shade ; 

 but L. incrustans thrives in the bright sunshine of shallow rock-pools. The 

 zone of L. Lenormandi occurs on damp rocks slightly above half-tide level, 

 at which height moisture is local, being found in fissures, on shady vertical 

 surfaces, or under the foliage of the larger algae. The zone is more extensive 

 than at first sight appears, and is probably widespread in the British Isles. 

 L. incrustans follows. This is abundant in all the rock-pools, and is by far 

 the commonest species in the district. Apart from its growth in pools, 

 L. incrustans first appears in chinks and hollows in the lower half of the 

 littoral region. As one descends the shore the crusts become larger and 

 more widely spread ; and on reaching the Himanthalia level they form large 

 sheets, filling up interstices between limpets, barnacles, and Ralfsia claxata. 

 At low-water mark the plant is joined by L. polymorphum, the red warty 

 thallus of which is at once distinguishable from the paler L. incrustans, 

 though the latter at this level assumes its true mauve-pink colour, in contrast 

 to its almost white appearance higher up. L. polymoiyhum extends into 

 the sub-littoral region ; L. incrustans probably does the same, but its growth 

 here on exposed shores was not investigated. 



The Lithothamnium vegetation described above is specially characteristic 

 of exposed, somewhat bare rocks, where algae such as Laurencia and 

 Himanthalia are poorly developed, but where there is a large growth of 

 the short form of Corallina. In localities where there is less exposure, or 

 where shrubby [and foliaceous algae occur in plenty, the Lithothamnium 

 vegetation is poor: some traces of the L. Lenormandi and L. polymorph urn 

 zones may be found, but L. incrustans usually disappears altogether. 



Vegetation of Rock-pools. 

 Several different types of pools may be distinguished according to their 

 elevation, and to the geological nature of the shore. A number of other factors 

 must also be taken into consideration — namely, the size, depth, and configu- 

 ration of the pools, and conditions of temperature, illumination, and drainage. 

 Whilst much of the pool-vegetation is perennial and more or less constant, 

 a large part is transient and varies considerably, not only from month to 



R. I. A. PKOC, VOL. XXXI. F 15 



