SCOTTISH FRESHWATER PLANKTON. 539 
other triangular form of this genus has been previously observed, 
viz., M. pinnatifida, Ralfs, var. trigona, West (in Journ. Bot. 
xxvi. (1888) p. 206, t. 291. f. 15). 
Genus Xanruipium, Ehren, 
~ 
5. X. antinopwum, Kutz. Spec. Algar. p. 177.—Cosmarium 
antilopeum, Bréb, in Linnea, xiv. (1840) p. 218. X. fascicu- 
latum, Ralfs, Brit. Desm. (ex parte), t. 20. f. La, ¢. 
The plankton-forms of this species are numerous and variable. 
Many of them are strictly typical, but more often they are not. 
One of the most abundant forms has the spines much reduced 
in size and number, some of the angles being occasionally quite 
destitute of spines. The semice)ls are also somewhat inflated. 
(Pl. 16. fig. 1.) 
Long. sine spin. 51-55°5 pw; lat. sine spin. 44-50 »; long. spin. 
2-9°5 w ; lat. isthm. 14°5-14 fs 
Hab. Loch Doon, Ayrshire. 
6. X. conrroversuM, West & G.S. West, in Journ. Linn. Soc., 
Bot. xxxiii. (1898) p. 298, t. 17. f. 2.—X. antilopceum ? var., 
est Y G. S. West, in Trans. Linn. Soc., ser. 2, Bot. v. (1896) 
p- 252, t. 16. f. 1. 
Var. PLANCTONICUM, var. n. (PI. 16. figs. 2, 3.) 
Var. major; sinu minus aperto, lineari, ad extremum ampliato ; 
semicellulis in centro scrobiculatis et cum tumore parvo; a 
vertice visis ellipticis cam tumore parvo in medio utrobique. 
Long. sine spin. 49-52; lat. sine spin. 46-49 « ; long. spin. 
16-23 p ; lat. isthm. 12°5-13°5 pw; crass. 31 pe. 
Hab. Loch Shin, Sutherland. 
This variety was extremely abundant from Loch Shin, and its 
characters are so striking that it might perhaps be regarded as 
a distinct species (X. planctonicum). The cells are larger than 
those of X. controversum, the sinus is closed, the spines are 
longer, and those of the inferior angles are less divergent ; in 
the centre of the semicells is a scrobiculated area which is 
thickened and shows as a protuberance in the vertical and 
lateral views. As in typical XY. controversum, odd spines of this 
variety are frequently replaced by a pair. 
It should be compared with X. tetracanthum, W. B. Turn. (in 
Kongl. Sv. Vet.-Akad. Handl. Bd. xxv. (1893) no. 5, p. 101, 
LINN, JOURN.— BOTANY, VOL. XXXV. 2s 
