28 BRITISH DESMID1ACEJE. 



possess any characters sufficiently definite to recognize them 

 as distinct types. There is, however, a type of this species 

 (var. trigibberum) which possesses characters by which it can 

 always be identified, and which must, therefore, be regarded 

 as a distinct variety. 



Brebisson described a " forma TRIQUETBA' of this species. 

 (Cousult Breb. ' Liste Desm/ 1856, p. 130, t. 1, f. 9; Eabeiih. 

 'Flor. Eur. Alg.' III, 1868, p. 171; Cooke, ' Brit. Desm/ 

 1887, p. 109; etc.) The cells in vertical view are triangular 

 with rounded angles, and there is a small inflation in the 

 middle of each concave side. Breadth 56-59 /,t. (PI. CI, 

 fig. 9.) This form has only been observed in France. 



Var. trigibberum Nordst. (PI. CI, figs. 10-15.) 



C. biretttm Breb. subsp. trigibberum Nordst. Desm. Arctoee, 1875, p. 26, 

 t. 7, f. 19 [inclus. C. biretum forma supernumeraries Nordst. 1. c. t. 7, 

 f. 18] ; Joshua in Journ. Bot. xxi, 1883, p. 2t)l ; De Toni, Syll. Alg. 

 1889, p. 1018 ; G. S. West, Variation Desm. 1899, p. 390, t. 10, f. 29-34 ; 

 W. & G. S. West, Alga-fl. Yorks. 1900, p. 88. 



Cells usually larger than in the type; semicells of 

 somewhat variable form, often more rounded ; vertical 

 view showing three small protuberances along each side. 



Length 43-100 p.; breadth 38-82 p. ; breadth of 

 isthmus 12-33/x; thickness 32-48 /A. 



ENGLAND. Roundhay Park, W. Yorks ! The Washes, 

 Button, Cambridgeshire ! Sutton Park, Warwickshire ! 

 Near Cirencester, Gloucestershire (Joshua). 



SCOTLAND.- -Keithick, near Coupar- Angus, Perth 

 (Roil $ Bissett). 



Geogr. Distribution.- -Spitzbergen. 



The distinguishing feature of this variety is the possession 

 of three protuberances on each side in the vertical view, a 

 central one and one close to each end. Considerable range 

 of variation is shown in the outline,, but we find the cells of 

 var. trigibberum to be more rounded in general contour than 

 those of typical C. birefum, and rarely to possess semicells 

 with divergent lateral margins. The three protuberances oil 

 each side of the vertical view may be slight and more or less 

 of equal size (PI. CI, figs. 101 and 136), or the central one 

 may be much larger than the other two, the latter varying 

 in their relative proximity to the ends (PL CI, fi's. 116 

 and 12 A). 



The granulation is precisely like that of the type. 



