98 BRITISH DESMIDIACE^. 



St. gracile is one of the most widely distributed and, at the 

 same time, most variable of Desmids, being variable both in the 

 form of its semicells, and the relative proportions of its " body ' 

 and processes. It is very closely allied to St. paradoxum Meyen, 

 and it is sometimes very difficult to distinguish between the 

 two species. The chief difference between them is that the 

 processes of St. gracile are horizontal or slightly converging in 

 the front view, whereas in St. paradoxum they are usually 

 shorter in proportion and divergent. The strength of the granu- 

 lation is also variable in this species. The granules are usually 

 stronger on the processes, and, if present on the "body" of the 

 semicell, they are very minute and little more than punctulations. 

 The plankton forms are often much larger than usual, and deviate 

 considerably from the typical form. The specimen figured on 

 PL CXLIV, fig. 6, is a peculiar plankton form with processes 

 tapering from a very broad base, and the apical granules so 

 strongly developed as to be almost spines. PL CXLIV, fig. 3 

 is a rather puzzling Desmid which was identified by Prof. G. S. 

 West as St. gracile in spite of its slightly divergent processes. 



Var. bicorne Bulnh. 



St. gracile var. bicorne Bulnh. in Hedwigia, 1861, p. 51, t. 9, f . 2 ; Arch, in 

 Quart. Journ. Micr. Sci. 1870, p. 86 ; Jacobs. Desm. Dane. 1875, p. 207 ; 

 De Toni, Syll. Alg. 1889, p. 1209 ; Roy & Biss. Scott. Desm. 1893, p. 20. 



Cells in front view similar to the type ; in end view 

 broadly fusiform, elongated at the poles to form pro- 

 cesses. 



Length 31 '3-32*5 p ; breadth, including processes, 

 52 '5-57 '5 ; thickness 15 (JL ; breadth of isthmus 12*5 pi. 



SCOTLAND.- -Upper Powlair, Slewdrum, Birsemore 

 Loch, Craigendinnie, Heughhead, west of the Ord, near 

 Loch Dawan, Homehead, Birkhill and Mosston Moor$ 

 Aberdeen ; Scolty Dam and Dalbrake, Kincardine ; 

 Fowlis Wester, Perth (Roy & Biss.). 



IRELAND. Mullingar, Co. Westmeath (Archer). 



Geogr. Distribution. Germany. Norway. Sweden. 

 Denmark. Finland. 



Var. bulbosum West. (PL CXLIV, fig. 13.) 



St. gracile subsp. bulbosum West, Alg. W. Ireland, 1892, p. 182, t. 23, f.ll. 

 St. gracile var. bulbosum West & G. S. West, Alg. N. Ireland, 1902, p. 54. 



