XANTHIDIUM. ) t 



X. subhastiferum is for the most part a plankton-Desmid, 



occurring now and then in considerable quantity in the 

 western lakes. It sometimes exhibits a certain amount of 

 irregularity in the disposition of the spines, but sucli irre- 

 gularities are merely slight monstrosities due to rapidity of 

 division and other causes. The normal specimen possesses 

 two equal divergent spines placed one above the other, but 

 occasionally one spine is reduced or absent (PL CVI. fig. 8), 

 or a third incipient spine is developed between the lateral 

 pair. 



It is a very distinct species, but should be carefully 

 compared with X. tetracentrotum Wolle. 



Var. Murray! W. & G. S. West. (PL CVI, figs. 10, 



11.) 



A', sulhastiferum var. Murrayi W. & G. S. "West, Scott. Freshw. Plankton, 

 I. 1903, p. 540, t. 16, f. 6 ; Further Contrib. Freshw. Plankton Scott. 

 Lochs, 1905, p. 485 ; Brit. Freshw. Phytoplankton, etc., 1909, p. 180 ; 

 Phytoplankton Engl. Lake District, 1909, pp. 138, 288. 



Semicells somewhat obsemicircular, with the apex 

 slightly convex but flattened in the middle, and with 

 the two lateral spines of each side disposed in a 

 horizontal (or sometimes more or less oblique) plane. 



Length 52-61 /x; breadth without spines 50*5-62 /.<, 

 with spines 87-9 7 p.; breadth of isthmus 17-19 p. 



ENGLAND. Plankton of Windermere, Hawes Water, 

 and Grasmere, Westmoreland ! 



SCOTLAND.- -Plankton of Loch Morar, Inverness ! ; 

 and Loch nan Cuinne, Sutherland (J. Murray) \ 



This variety seems to he confined to the plankton, and since 

 its first discovery in Loch nan Cuinne, Sutherland, it has been 

 found in considerable abundance in other Scottish nnd English 

 lakes. It is one of the leading constituents of the autumn 

 plankton of Windermere, occurring in quantity from 

 September to November. 



A slight abnormality was observed in a single specimen 

 from Grasmere in the English Lake District, one semicell 

 possessing an obliquely disposed pair of short, curved, apical 

 spines (vide PI. GUI, hg. 14). 



A triangular form, which we have named "forma TRIQUETRA* 

 occurs in the plankton of Loch Lomond (vide PL CVI1, figs. 

 9, 10). This differs only in the triangular character of the 



