XANTHIDIUM. 67 



straight or very faintly convex sides, each of which 

 is slightly thickened in the middle, angles wide and 

 emarginate, furnished with a pair of stout spines, 

 and within each angle with a similar pair of stout 

 spines. Each semicell with 6 parietal chloroplasts. 



Length without spines 81-90 /*, with spines 116- 

 126/x; breadth without spines 65-82 /A, with spines 

 97-125 //,; breadth of isthmus 26-28'5/x. 



ENGLAND.- -Plankton of Brothers' Water, Grasmere, 



and Easdale Tarn in the En High Lake District ! 



~ 



Geogr. Distribution. Sweden. Finmark in Xor- 

 way. Austria and Gralicia. Brazil (forma). 



In the British Islands we have only observed this variety 

 in the plankton. Lutkemuller has recorded smaller forms 

 from Austria; length without spines 71^u, with spines 90 /i; 

 breadth without spines 55 JJL, with spines 84 /(. 



Var. polymazum Kordst. (PL CVIII, fig. 19.) 



X. antilopaeum var. polymciziim Nordst. Norges Desm. 1873, p. 38, t. 1, 

 f. 19; Wolle, Desm. U.S. 1884, p. 94, t. 23, f. 3, 4 [figures poor]; 

 Borg. Bornholm Desm.-fl. 1889, p. 149 ; W. & O. S. West, Further 

 Contrib. Freshw;. Plankton Scott. Lochs, 1905, p. 485 ; Cushman in 

 Rhodora, vii, 1905, p. 259 ; W. & G. S. West, Brit. Freshw. Phyto- 

 planktoii, etc., 1909, p. 180. 



Each semicell with a semicircular series of large 

 rounded granules above the central protuberance. 



Length without spines 46-68 /x, with spines 6285 p. ; 

 breadth without spines 45-62 //,, with spines 58-76*5 ^ ; 

 breadth of isthmus 12-1 6 IJL; thickness 28-33 /x,. 



SCOTLAND.- -Plankton of Loch Ghriar, Sutherland ; 

 Loch Fadaghoda, Lewis ; and Loch nan Eun, X. TJist, 

 Outer Hebrides ! 



Geogr. Distribution.- -Xorwaj. Bornholm. United 

 States. 



This is a rare variety in the British Islands and one that 

 we have found only in the plankton. In the original figure 

 given by Nordstedt there is an incipient third spine between 

 the pair at each lateral angle of the semicells, and we often 

 find this spine fully developed so that there are three equal 

 spines at each lateral angle (consult PI. CIX,, fig. 2). 



