182 BRITISH DESMIDIACE^E. 



A careful consideration of the various forms of St. punc- 

 tulntnm and its allies has convinced us that St. Kjellmani 

 Wille, St. pygmaeum Breb. and St. punctulatum all belong to 

 the same species-group. We have found it clearly impossible 

 to separate these Desmids as distinct species, as the inter- 

 mediate forms constitute a gradation through which any line 

 of demarcation is purely arbitrary. We have therefore placed 

 all the various forms under the one species St. punctulatum. 



We wish it to be understood that this closer grouping of 

 the above " species," so often regarded as distinct, is an 

 expression of opinion that after twenty years 7 experience of 

 them from all parts of the world we are still unable to draw 

 up valid specific characters. 



St. subpunctulatum Gay (' Monogr. loc. Conj. Montpellier,' 

 1884, p. 66, t. 2, f. 8) is most probably only a form of St. 

 punctulatum. 



It is possible to discriminate between seven more or less 

 distinct British varieties of St. punctulatum. 



Var. subpro due turn var. nov. (PL CXXVII, fig. 15.) 



Staurastrum punctulatum forma G. S. West, Alga-fl. Cambr. 1899, p. 219. 



Cells proportionately wider, lateral angles of semi- 

 cells very slightly produced ; vertical view with faintly 

 convex sides and almost imperceptibly produced 

 angles. 



Length 31/i; breadth 31 ^ ; breadth of isthmus 

 8*5 p. 



ENGLAND.- -Dernford Fen, Cambridgeshire ! 



To this variety belongs the form described by Boldt from 

 Greenland (</. Boldt, 'Desmid. Gronland/ 1888, p. 35, t. 2, 

 f . 43) as "forma semicellulis a vertice visis area triangulari 

 nuda, lateribus in medio tumidis." This form has the faintly 

 produced angles characteristic of var. sub product urn, but in the 

 middle of the apex of each semi cell is a triangular smooth area. 



Var. Kjellmani Wille. (PI. CXXVII, figs. 13, 17-19, 



21, 22.) 



Staurastrum Kjellmani Wille, Ferskv. Alg. Nov. Semlj. 1879, p. 50, t. 13, 

 f . 50-53 [forma trigona major, forma trigona minor, and forma tetra- 

 gona] ; Cooke, Brit. Desm. 1887, p. 163, t. 54, f . 9 ; West, Alg. N. 

 Yorks. 1889, p. 293 ; Alg. W. Ireland, 1892, p. 177 ; Alg. Eiigl. Lake 

 Distr. 1892, p. 731 ; Roy & Biss. Scott. Desm. 1894, p. 21 (sep.) ; Nordst. 

 Index Desm. 1896, p. 152 ; W. & G. S. West, Alga-fl. Yorks. 1901, p. 

 102 ; Bohlin, Flor. Algol, d'eau douce d. A9ores, 1901, p. 62 ; Lai-sen, 



