16 138 Proceedings of the Royal Irish Academy. 



disappears, and the plant assumes a green colour, pale when'growing in sunny 

 pools, but darker when on bare rocks or under the shade of other plants. It 

 should be noted that L. pinnatifida also reaches its maximum development in 

 spring ; but it is present more or less throughout the summer, and appears to 

 possess a persistent base (see p. 35). 



L. hybrida has been also confused with L. dbtusa with which it agrees in 

 possessing cylindrical fronds. The colour and habit of the two are, however, 

 very distinct. L. dbtusa is a sub-littoral plant of dark red colour which becomes 

 yellow with sunlight. It is also found in pools and shallow water, especially 

 in winter and spring, and is almost always epiphytic. There is further an 

 important morphological distinction to be noted in the structure of the 

 antheridial conceptacles. The conceptacles of L. hybrida will be found to 

 resemble those of L. pinnatifida, and to be quite distinct from L. obtusa 

 (see Falkenberg, '01, pp. 242 and 249). 



The distribution of the present species is more restricted than that of 

 the other two. It is common in England, Scotland, and Ireland, and is 

 found on the north and west coasts of France ; but I have not seen undoubted 

 specimens from other countries. The records from the Mediterranean, Ceylon, 

 Japan, and the West Indies require verification. The antheridial character 

 referred to above will possibly aid in the identification of this species, since 

 in Laurencia exact similarity of form cannot be insisted upon when climatic 

 conditions are different. 



Polysiphonia macrocarpa Harv. 



First described by Harvey in Mackay's " Flora Hibernica," part ii, p. 206. 

 It was wrongly united in " Phyc. Brit." with P. pulvinata, but is rightly 

 presented in Batters ('02). Amongst British species it is very distinct; 

 but Bornet suggested that it might be a form of the Mediterranean 

 P. sertularioides, though in his "Algues Sehousboe" he preferred to keep them 

 apart. P. macrocarpa usually grows in shallow crevices near low- water on 

 exposed rocks, being not uncommon on Clare Island, and the only species 

 of the genus noticed on The Bills. 



P. fastigiata Grev. 

 Though usually confined to Ascophyllum nodosum, it is interesting to note 

 that in exposed localities (where that plant is absent) P. fastigiata is frequently 

 found on Fucus spiralis. In a general way F. spiralis on exposed rocks on 

 Clare Island is free from the epiphyte ; but at Alnahaskilla scores of infested 

 plants may be counted. P. fastigiata was noted on the same host at other 

 spots on the island, and in one case it occurred on F. vesiculosus. 



