15 106 Proceedings of the Royal Irish Academy. 



gradually or abruptly into the stalk ; but in any one gathering many 

 intermediate forms are found. It agrees well in form with Braun's specimens 

 (Eabenhorst, " Alg. Eur.," No. 1841), but is somewhat smaller. It also 

 appears to agree with the Berwick gatherings (Holmes, " Algae Rariores," 

 No. 33), which Batters ('89, p. 44) referred to C. gregariwm, but which, in 

 1902, he placed under C. pusillum Foslie. Judging by Borgesen's figure 

 of the original material ('02, fig. 107), it would appear that Batters' earlier 

 view was the more correct. The specimens of C. pusillvm distributed 

 by Wittrock and Nbrdstedt, and by Hauck and Biehter fExsicc, Nos. 457 

 and 472 respectively), agree with Borgesen's figure in possessing an extremely 

 long head, and such specimens are easily distinguished from the normal form 

 of C. gregarium. 



With regard to the other species, the Clare Island observations tend 

 to show that names have been proposed far too freely ; and it is clear that 

 nothing but a special study with careful and systematic collecting throughout 

 the year will bring about a true understanding of the genus. 



MONOSTROMA. 



In common with many algae of brackish regions the Monostromas 

 present endless trouble to the systematist ; but it would seem that few 

 of these genera would so readily repay careful study in the field. The 

 characters upon which the species are founded — the size, shape, and thickness 

 of the frond, and the size and form of the cells — are known to vary much 

 with external conditions. If the ecology of the plants could be studied, and 

 their growth traced through from the earliest attached stages to the loose 

 floating forms, it is certain that many puzzling intermediates could be placed, 

 and that several of the so-called species would have to disappear. 



Of the numerous forms noted in Clew Bay, M. orbiculatum was recognizable ; 

 and two others that were sufficiently distinct and frequent to be worthy of 

 record, could not be satisfactorily determined, and are hence referred to by 

 collecting-numbers only. 



M. orbiculatum Thur. 



A plant common during April and May in the brackish pools of the salt- 

 marsh near Belclare. It is at first attached to stones, sticks, or leaves 

 and shoots of flowering plants, but finally becomes free. It often grows 

 in dense tufts, and may reach as much as 1 foot to 18 inches across. The 

 fronds are densely plicate, usually orbicular, but sometimes partly sub- 

 lanceolate ; on detachment they become very irregular, but, owing to 

 spring-tides flooding the pools, it is difficult to be certain as to the origin 

 of free-swimming plants. 



