84 BRITISH DESMIDIACEJE. 



IRELAND. Near (Plenties, and Glendoan, Donegal ! 

 Armagh (Archer). Kilclare (Crowe). Glengariff, Cork 

 (Crowe). 



As in the case of so many other small Desmids discovered 

 by Archer, the promised full description of X. Robinson ianum 

 did not appear, so that the only information concerning it are 

 the brief statements in the reports of the Dublin Microscopical 

 Club. It is a very rare Desmid, which up to the present has 

 been found nowhere in the world except in the western areas 

 of the British Islands. Its exact identity was long uncertain, 

 which accounts for the fact that we described it in 1902 as 

 ' X. variabile var. complexwn.' Careful consideration, how- 

 ever, of the meagre information supplied by Archer, convinces 

 us that the Desmid we have so described is X. Rol>inxon ianum, 

 and that another minute Xanthidium which we had described 

 under the name of X. Robinaonianum is an undescribed 

 species. This view was amply confirmed by an examination 

 of the specimens issued in Wittrock and Nordstedt's ' Alga3 

 Exsiccata?/ No. 550, under the name of X. Robinsonianum. 

 These are undoubtedly the Desmid which we described as 

 X. variabile var. complexum and which we have since come 

 to regard as X. Robinsonianum. We give some figures of 

 these specimens (PI. CXVIII, figs. 1-3) for comparison with 

 those of specimens collected by ourselves. 



In some individuals the central protuberance is furnished 

 with three elongated and regularly disposed granules, but in 

 others there are four or five smaller granules, and these are 

 not infrequently connected with the small group of subapical 

 spines by several acutely-pointed granules. 



X. Robinsonianum differs from X. variabile in the shape of 

 its semicells, in the closed sinus, and in the more numerous 

 and differently grouped spines. The zygospore is also 

 probably of a different character, but Joshua's remarks on 

 the characters of the zygospore are very indefinite and he 

 gives no figures. The zygospores were obtained from Derry- 

 strasna Bog, Armagh. 



13. Xanthidium Orcadense sp. noc. 

 (PL CXIII, figs. 10-12.) 



Xanthidium Robinsonianum W. & GK S. West, New and Int. Fresliw. Al<^. 

 1896, p. 156, t. 3, f. 21, 22. [This is not X. Robinsonianum Arch.] 



Cells small, as long as broad, deeply constricted, 

 sinus open and acute-angled ; semicells transversely 



