15 112 Proceedings of the Royal Irish Academy. 



branches are more numerous.- These are inseparable from Acrosiphonia 

 albescens Kjellm.,.and A. incurva Kjellm., which Borgesen found so abundantly 

 in the Faeroes. C. Traillii Batters is also hardly distinct from 0. arcta. All 

 these forms appear to exist on Clare Island ; but they pass imperceptibly 

 into one another. The species vary greatly with age, and with habitat, so 

 that for a satisfactory revision it is essential that much time be spent in 

 field work, 



Derbesia marina Kjellm. 



Two patches of this plant were found — (1) in a cave near Portlea, and 

 (2) on the dark under-side of a slab, at Kinnacorra, attached to Haliclwndria 

 panicea. In the latter case the alga formed tufts about 1 cm. high of a pale 

 green colour. At the base there is a mass of creeping filaments, and from 

 these spring upright shoots with numerous lateral branches coming oh at an 

 acute angle. The latter are much more slender than in the Finmark and 

 Faeroese plants, being only 20-30/z diam., but they agree in possessing the 

 short cells at the base. 



The cave-specimens (which grew in or at the margins of shallow pools) 

 were much less regular in growth, the branches springing from the main 

 filaments at various angles, but they agreed in size and in presence of the 

 short cells. The matted growth at the base is more copious and the upright 

 filaments are shorter than in the plants from the open. Sporangia were 

 found on the cave-specimens only. These were few in number and hardly 

 mature ; the largest measured 120-65/x, which is smaller than those recorded 

 previously. 



Kjellmann described D. manna (Lyngb.) in full ; and Borgesen in dealing 

 with its distribution records the plant from Lerwick ('02, p. 520, '03, p. 303). 

 Up to the present this has been the only certain record for the British Isles. 

 It appears probable from the Portlea specimens that Crouan's D. repens may 

 be a form of the present species. 



Vaucheria Thuretii Woron. 



This species occurs half-buried in the mud on the lowest part of the shore ; 

 and it covers wide areas of the mud-fiats in Clew Bay and Achill Sound (north 

 end). It is distinguished by numerous small sessile antheridia found in the 

 neighbourhood of each oogonium and on the same filament. At times the 

 growth is so covered with mud as to be hardly visible, but even in this 

 state it provides, on washing, excellent fruiting material. Found in fruit. 

 May, July, September, at Annagh Island, Mulranny, and Achill Sound. (See 

 p. 04. , 



