70 BRITISH DESMIDIACE.E. 



Var. depauperatum W. & G. S. West. (PI. CX, 



figs. 3-7.) 



X. antilopaeum forma W. & G-. S. West, Scott. Freshw. Plankton, I. 1903, 



p. 539, t. 16, f. 1. 

 X. antilopseum var. depauperatum W. & G. S. West, Freshw. Alg. Orkneys 



and Shetlands, 1905, p. 23, t. 1, f. 15, 16; Further Contrib. Freshw. 



Plankton Scott. Lochs, 1905, p. 485 ; Comp. Study Plankton Irish 



Lakes, 1906, p. 85 ; Brit. Freshw. Phytoplaiikton, etc., 1909, p. 180 ; 



Phytoplankton Engl. Lake District, 1909, p. 138. 



Semicells slightly inflated, with the lateral angles 

 very obtuse and generally very slightly truncate ; 

 spines 13 (more rarely 4) on each lateral margin, 

 very short, of unequal length, and irregularly disposed ; 

 central area of normal size and closely scrobiculated, 

 or with only one ring of scrobiculations. 



Length without spines 46-55 '5 JJL ; breadth without 

 spines 43-50^; length of spines 2-10*5 /x; breadth 

 of isthmus 9'5-14/x. 



ENGLAND.- -Plankton of Buttermere, Crummock 

 Water, and Ennerdale Water, Cumberland ! ; and of 

 Red Tarn, Grasmere, and Easdale Tarn, Westmore- 

 land ! 



WALES.- -Plankton of several Welsh lakes ! 



SCOTLAND.- -Plankton of Lochs Griama and nan 

 Cuinne, Sutherland ; of Loch na Cloiche Sgoilt, Inver- 

 ness; and of Loch Tay, Perth! Plankton of lakes in 

 Orkneys and Shetlands ! 



IRELAND. Plankton of lakes of Gal way and Kerry ! 



In this variety the semicells are less hexagonal and more 

 inflated than in the typical form. The spines are fewer, thinner, 

 of unequal length, and show much irregularity in their dis- 

 position. In fact, the number and disposition of the spines is 

 generally quite different on the two semicells of the same 

 individual. It is sometimes a distinctive feature of the lake- 

 plankton. 



7. Xanthidium cristatum Breb. 

 (PI. CX, figs. 8, 9; PL CXI, fig. 1.) 



Xanthidium cristatum Breb. in Kalfs' Brit. Desm. 1848, p. 115, t. 19, f. 3 

 a-c- Arch, in Pritch. Infus. 1861, p. 736, t. 2, f. 18, 23; Kabenh. 

 Flor. Europ. Algar. Ill, 1868, p. 224; Lund. Desm. Suec, 1871, p. 76; 

 Keinsch, Contrib. Alg. et Fung. 1875, p. 92, t. 16, f. 13 ; Kirchn. Alg. 



