XANTHIDIUM. 77 



that Nordstedt's var. ornatnm (from Finland, Sweden, and 

 Greenland) is nothing more than a normally developed form. 



The confusion between this species and X. antilopseum is 

 not confined to the writings of older authors,, but is equally 

 manifest in the publications of many modern investigators. 

 \_cide remarks by Borgesen, Larsen, etc.] 



An American variety var. Oronense W. & G-. S. West- 

 from Orono, Maine, which we figure (PI. CXI, fig. 9) for com- 

 parison with the typical form, also possesses a granulated 

 central area and the six equidistant pairs of spines on each 

 semi cell. 



In the British Islands X. fasciculatum is a very much rarer 

 Desmid than X. antilopseum. 



9. Xanthidium Brebissonii Ralfs. 

 (PI. CXII, figs. 1 and 3.) 



BinatcUa acideata Breb. Alg. Falaise, 1S35, p. 58, t. 8. 



Xanthidium Brebissonii Ralfs, Brit. Desm. 1848, p. 113, t. 19, f. 2 a, d ; 

 Arch, in Pritch. Infus. 1861, p. 736; Eabenh. Flor. Europ. Alg. Ill, 

 1868, p. 223 : Cooke, Brit. Desm. 1887, p. 130 ; De Toiii, Syll. Alg. 1889, 

 p. 926 ; West, Alg. N. Wales, 1890, p. 291 ; Alg. Engl. Lake District, 

 1892, p. 730 ; Roy. & Hiss. Scott. Desm. 1893, p. 244 ; Nordst. Index 

 Desm. 1896, p. 69 ; W. & G. S. West, Alga-fl. Yorks. 1900, p. 68; Borge 

 Beitrage Alg. Schweden, 1906, p. 28. 



Holacanthum Brebissonii Migula, Kryptoganienfl. in Flora Deutschl., 

 Osterreich u. Schweiz, Hd. 5, 19u6, p. 516, t, 28, f. 6. 



Cells of medium size, about as long as broad (without 

 spines), deeply constricted, sinus almost closed at the 

 apex but opening outwards ; semicells subsemicircular, 

 base slightly convex and apex a little flattened in the 

 middle, furnished with 8 or 10 pairs of stout marginal 

 spines, central area strongly protuberant and truncate, 

 furnished with a circle of 12-13 prominent granules. 

 Side view of semicell subcircular, with a truncate 

 granulated protuberance at the middle on each side, 

 and 2 or 3 spines at the apical margin. Vertical view 

 elliptic, with 2 or 3 spines at each pole, and a truncate 

 granulated protuberance at the middle on each side. 



Zygospores unknown. 



Length without spines 61-80 /x, with spines 79- 

 101 jit; breadth without spines 62-76 /*, with spines 

 80-104 JLL; breadth of isthmus 18-27 ft; thickness 

 36-50 ft. 



