15 118 Proceedings of the Royal Irish Academy. 



Biology and Ecology. — The bright green colour referred to is an optical 

 effect due to the presence of air-bubbles between the utricles. Two explanations 

 suggest themselves : ( 1 ) A physiological disturbance in the tissues due to the 

 heating of the pools, and (2) the retention of oxygen given off during 

 photosynthesis in bright sunlight. Plants growing in the shade are dark in 

 colour, as were those collected in February. But in April, when there is 

 sufficient sunlight to cause heating and rapid photosynthesis, the bright green 

 appearance is, on the south shore of Clare Island, everywhere in evidence. At 

 the Giant's Causeway, however, where the coast faces north, a few plants 

 collected on May 1 were still dark. It is noteworthy that when C. tomcntosum 

 grows in sunny pools, which is very rarely the case, no gas is found between 

 the utricles, and the normal colour is retained. 



The habitat of our plant is well marked. It is distinctly an alga of sunny 

 rock-pools, being found at any level, but more commonly at a quarter to three- 

 quarter tide. It also occurs occasionally in the shallow sub-littoral region, or 

 on bare rocks near low-water mark, where, though liable to be exposed to the 

 air during the lowest tides, it may attain a large size. In this position it is dark 

 in colour. In a general way the plant is frequent on the south shore of Clare 

 Island; also at Roonah and Dog's Bay. It prefers moderately open rocky 

 ground, but extends into Clew Bay as far as Old Head. Several plants were 

 noted in the quiet, brackish waters of Bellacragher Bay ; but they were 

 decidedly unhealthy, the fronds being not only irregular in form, but 

 exhibiting abnormalities in microscopic structure. The utricles were for the 

 most part strongly mucrouate, and showed a tendency to become irregularly 

 thickened at the apex. 



C. mucronatum var. atlanticum appears to be an annual. On the February 

 visit young plants only were observed, and large plants are prevalent in 

 summer, though a few sporelings may be found all the year round. 

 0. tomcntosum, on the other hand, attains its maximum size in winter. 



Distribution. — The remarkable feature with regard to distribution is not so 

 much its link with the Australian forms (our other species, C. tomcntosum and 

 C. adhaerens, apparently occur in the Southern Hemisphere), but its isolated 

 position in Europe. It is not known from the Mediterranean or from North 

 Africa, and is apparently entirely absent from the remainder of the North 

 Atlantic. Dr. Marshal Howe, who has collected largely on the American 

 coasts and in the West Indies, writes in reply to an inquiry, that as far as 

 he is aware '' no mucronate Codium has ever been found on the Atlantic 

 coast of North or South America, or on any of the outlying islands." 



The distribution in the British Isles is given on p. 115. No English or 

 French specimens have been found in any of the herbaria examined ; neither 



