u 



124 BRITISH DESMIDIACEJ:. 



at once distinguishes it from all other species of the genus. 

 The commonest forms are pentagonal in vertical view, 

 although tetragonal forms are by no means infrequent. 



A form in which the apex of the semicells is more elevated 

 has been described by Borge from Norway (Fin mark) as 



forma rotundata 3 (Borge, ' Chlorophy. Norska Finmark/ 

 1892, p. 7, t. 1, f. 4). 



2. Staurastrum Capituhim Breb. 



(PL CXVIII, figs. 7 and 10.) 



Staurastrum Capitulum Breb. in Ealfs' Brit. Desm. 1848, p. 214, t. 35, 



f. 25 [figure inaccurate] ; Arch, in Pritch. Infus. 1861, p. 740; Eabenh, 



Flor. Europ. Alg. III,"l868, p. 209 ; Kirchn. Alg. Schles. 1878, p. 165 ; 



De Toni, Syll. Alg. 1889, p. 1191 ; West, Alg. N. Wales, 1890, p. 294; 



Borge, Sussw. Chlor. Archang. 1894, p. 37 ; Nordst, Index Desm. 1896, 



p. 72. 

 St. amoenum Hilse in Ber. d. Schles. Gesellsch. 1865 [1866], p. 123 ; 



Kirchn. Alg. Schles. 1878, p. 165 ; Wille Ferskvandsalg, Nov. Semlj. 



1879, p. 54 ; De Toni, Syll. Alg. 1889, p. 1191 ; Heimerl, Desm. alp. 



1891, p. 606 ; West, Alg. W. Ireland, 1892, p. 179 ; Alg. Engl. Lake 



Distr. 1892, p. 732. 

 St. Capitulum b. amoenum Eabenh. Flor. Europ. Alg. Ill, 1868, p. 209; 



Cooke, Brit. Desm. 1887, p. 161 ; W. & G. S. West, Alga-fl. Yorks. 1901, 



p. 107. 



Cells rather small, about 1^-lf times as long as 

 broad, slightly constricted, sinus a small acute-angled 

 notch ; semicells campanulate from a broad base, 

 which, is slightly tumid on each side and 2-4-denticu- 

 late (this marginal appearance being caused by a ring* 

 of denticulate verrucas completely encircling the base 

 of the semicell), sides slightly retuse above the base 

 and then upwardly divergent, apex broad and faintly 

 retuse, apical angles generally denticulate with sharp 

 granules arranged in concentric rings ; granules 

 diminishing in size and sharpness as they extend 

 towards the basal ring of verrucae. Vertical view of 

 basal part of semicell circular with a crenate margin ; 

 crenae 10-12, denticulate or emarginate; vertical 

 view of apical part triangular with subacute angles 

 and almost straight sides. 



o 



Zygospore unknown. 



Length 37--40/Z; breadth of base of semicell 21- 

 22 /x, of apex 26-28^; breadth of isthmus lo-14'o/z. 



