81 



SCOTLAND.- -Near Dinnet, Aberdeen; Tent's Moor, 

 Fife (Rot/ $ JHwtt). 



Geofjr. Distribution.- -Qerm&ny. Austria. Bornliolm. 



Tin's variety has been erroneously referred to X. Brebissonii, 

 whereas the form of the sernicells, the irregularity of the 

 spines, and the nature of the central protuberance are all 

 much more in agreement with X. aculeatuin. Moreover, 



O - 



the semicells possess that raised apex which is not infrequently 

 met with in X. aculeatum, but never in X. Br< : hl*x<nrii. The 

 semicells of the latter species are proportionately wider and 

 the spines are much more regularly disposed. 



11. Xanthidium variabile (Xordst.) W. & Gr. S. West. 



(PI. CXIII, figs. 1-7.) 



X. Smithii Arch. var. variabile Nordst. in Botan. Xotis. 1887, p. 159; 



Freshw. Alg. N. Zeal. 1888, p. 44, t. 4, f. 27-29 ; West, Alar. W. Ireland, 



1892, p. 166 ; W. & G. S. West, New and Int. Freshw. Alg. 1896, p. 



156, t. 4, f. 40; Alg. S. England, 1897, p. 484; G. S. West, Variation 



Desm. 1899, p. 386, t. 8, f. 20-22. 

 A. cut-labile (Nordst.) W. & G. S. West, Notes Alg. II, 1900, p. 291; 



Alga-fl. Yorks. 1900, p. 68 ; Alg-. N. Ireland, 1902, p. 31 ; Freshw. Alg. 



Ceylon, 1902, p. 160. 

 X. irestianum Gutw. Alg. Ins. Java, 1902, p. 590. 



Cells small, about as long as broad, or a little 

 longer, deeply constricted, sinus open and acute- 

 angled ; semicells rectangular-trapezoid, sides and 

 apex almost straight, angles slightly rounded and 

 furnished with 1-- 4 (commonly 3) very short, sharp 

 spines (one of which is usually seen within the 

 margin), central area small and strongly protuberant, 

 usually almost papilliform, but sometimes broader 

 and triangulate or even bispinate. Side view of 

 semicell angular-subcircular, with a protuberance 

 (sometimes emarginate or bispinate) at the middle 

 on each side, and a small spine at each side of the 

 apical margin. Vertical view elliptic, with a papilli- 

 form (or emarginate) protuberance at the middle on 

 each side, poles rounded or subtruncate, trispinate 

 (or rarely only bispinate). Cell- wall smooth. Chloro- 

 plasts axile, one in each semicell, with a central 

 pyrenoid. 



VOL. iv. 6 



