EUASTEUM. 17 



Faeroes. Greenland. India. Ceylon. Azores. United 



t/ 



States. Paraguay. 



This is one of the most abundant of the laro-e British 



o 



species of Euastrum, and is subject to considerable variation 

 in outward form. The sinuation of the lateral margins of the 

 semicells varies very much, even in the semicells of the same 

 plant. The form named " var. sinuatum ' ' by G-ay has there- 

 fore no claim to rank as a distinct variety. The form named 

 " var. tatricum" by Eaciborski is based on an error in Ralfs' 

 ' British Desmids. 1 This is another species in which Ralfs 

 failed to depict the surface protuberances correctly. The 

 only protuberances on the semicell of any specimen of 

 E. Didelta are three across the base, the middle one beino- 



' ~ 



immediately above the isthmus, and two across the centre. 



Monstrosities of this Desmid are not uncommon ; they have 

 been figured by Archer in ' Proc. Dubl. Nat. Hist. Soc./ 1859, 

 p. 208, t. 1, f. 12; ' Nat. Hist. Rev./ 1859, vi, p. 470, t, 33 ; by 

 Bulnheim in ' Hewidgia/ 1861, p. 52, t. 9, f. 6; and by West 

 & G. S. West in ' Ann. Bot,/ xii, 1898, t. iv, f. 39. 



Some specimens possess a large scrobiculation beneath the 

 two central protuberances ; this form has been named " forma 

 scrobiculata Nordst.," and is fairly general in its distribution. 



7. Euastrum affine Ralfs. 

 (PL XXXV, figs. 11,12.) 



Euastrum affine Ealfs in Ann. Mag. N"at. Hist, xiv, 1844, p. 191, t. 7, f. 3 ; 

 Hass. Brit. Freshw. Alg. 1845, p. 382, t. 90, f . 9 ; Kalfs Brit. Desm. 

 1848, p. 82, t. 13, f. 3; Arch, in Pritch. Infus. 1861, p. 729 ; Eabenh. 

 Flor. Europ. Alg. Ill, 1868, p. 182 ; Cooke, Brit. Desm. 1886, p. 67, 

 t. 33, f . 2 ; De Toni, Syll. Alg. 1889, p. 1091 ; West, Alg. W. Ireland, 

 1892, p. 137 ; Alg. Eng. Lake Distr. 1892, p. 722 ; Roy & Biss. Scott, 

 Desm. 1893, p. 175; Xordst. Index Desm. 1896, p. 40; West & G. S. 

 West, Alg. S. England, 1897, p. 483 ; Alga-fl. Yorks. 1900, p. 61 ; Alg. 

 N. Ireland, 1902, p. 28. 



Helierella affinis Kuntze, Eevis. gen. plant. 1891, p. 898. 



Cells of medium size, somewhat less than twice as 

 long as broad, very deeply constricted, sinus narrowly 

 linear with a dilated extremity ; semicells three-lobecl 

 with two intermediate lobules, sinuses between the 

 lobes and the lobules deep and widely open ; polar lobe 

 outstanding, anvil-shaped, with rounded angles and a 

 convex apex, and with a median linear notch of some 



2 



