52 BRITISH DESMIDIACE^E. 



1901, p. 229; "VV. & G. S. West, Alg. N. Ireland, 1902, p. 30; Him, 

 Desm. Finland, 1903, p. 23 ; W. & G. S. West, Scott. Freshw. Plank- 

 ton, I. 1903, p. 527 ; Cushm. in Bull. Tor. Bot. Club. 1904, xxxi, p. 583, 

 t. 26, f. 7 ; W. & G. S. West, Further Contrib. Freshw. Plankton Scott. 

 Lochs, 1905, p. 485 ; Corap. Study Plankton Irish Lakes, 1906, p. 85 ; 

 Borge, Beitrage Alg. ScliAveden, 1906, p. 28 ; W. & G. S. West, Brit. 

 Freshw. Phytoplankton, etc., 1909, p. 180 ; Phytoplankton. Engl. Lake 

 Distr. 1909, p. 138. 



Euastrum Eigorrianiim Perty in Mittheil. d. naturforsch. Ges. Bern, 

 1849, p. 174. 



Zygoxanthiuin Bigorrianum Perty, Kleinst. Lebensf. 1852, p. 209, t. 16, 

 f. 23. 



Didymidium (Xanthidiwm) armatum Eeinsch, Algenfl. Frank. 1867, p. 129. 



Xanthidium Bigorrianum (Perty), Eabenh. Flor. Europ. Alg. Ill, 1868, 

 p. 222. 



X. armatum var. Wolleanum Turn. New and Eare Desm. 1885, p. 938, 

 t. 15, f. 18. 



X. armatum var. Americanum Turn. 1. c. t. 15, f. 19. 



Schizacanthum armatum (Breb.) Wille in Engler & Prantl, Natiirl. Pflan- 

 zenfamil. 1890, p. 11, 12, f. 7E. 



Xanthidium armatum var. supernumerarium Schniidle, Beitr. alp. Alg. 

 1895, p. 348, t. 15, f. 8. 



Cells large, from 1-J- to 1-J times as long as broad, 

 deeply constricted, sinus open and acute-angled, some- 

 what acuminate at the extremity ; semicells somewhat 



V 



octangular (reckoning the isthmus as one of the eight 

 sides), lower margins slightly convex, inferior and 

 superior lateral margins usually very slightly concave 

 but not infrequently straight, apex straight or 

 slightly convex, with the two lateral angles and the 

 apical angle on each side furnished with short, stout, 

 solid, wart-like spines, which are 2-4 (commonly 8)- 

 furcate at the apex, with a similar stout furcate spine 

 within each lateral angle and with a pair just within 

 the apex (all of which are subject to variation), in the 

 centre of the semicell with a ring of simple or emar- 

 ginate teeth representing the serrated edge of a large 

 central wart. Side view of semicell ovate-rectangular, 

 near the base on each side with a large truncate toothed 

 wart, apical angles furnished with 2-8-furcate spines, 

 and with 2 similar furcate spines at the truncate apical 

 margin. Vertical view elliptic-oblong with subtruncate 

 poles, with a large truncate toothed wart at the middle 

 on each side, and with four stout 2-3 -furcate spines at 

 equal intervals along each polar margin. Cell- wall 

 punctate. Chloroplasts sometimes distinctly parietal, 



