32 BRITISH DEvSMIDIACE2E. 



ENGLAND. E slier West-end Common, Surrey ! 



SCOTLAND. In the plankton of Locli Fadaghoda, 

 Lewis, and Locli Laxadale, Harris, Outer Hebrides ! 



IRELAND.- -Plankton of Loughs Caragli, Currane, and 

 Guitane, Kerry ! 



Geogr. Distribution. Sweden. N. Russia. Green- 

 land. Australia (form). Argentina. 



This minute species, which sometimes occurs in quantity in 

 the plankton, is a very near relative of several others, from 

 which it is necessary to distinguish it. From C. arctoum 

 Nordst. it is distinguished by its deeper constriction and the 

 narrower apices of the semicells; from C. bioculatum Breb. 

 by its shallower constriction and slightly flattened apices ; 

 from C. tinctum Ralfs by its proportionately broader cells, its 

 more depressed semicells, its more delicate and colourless 

 cell-wall. In the breadth of its isthmus it resembles 

 C. asphderosporum Nordst. (consult vol. ii, p. 163, pi. Ix, figs. 

 24, 25), but differs from that species in the more convex 

 apices and in the elliptical vertical view. 



Another closely allied species, which would perhaps be 

 better considered as a form of it, is C. affiue Racib. (' Desmidyja 

 Ciastonia/ 1892, p. 363, t. 6, f. 25). 



Nordstedt has recorded a very minute form from Sweden : 

 length 12 u ; breadth 10 fi ; breadth of isthmus 7 /LL ; thickness 

 6 fji (vide C. globosum Bulnh. *subarctoum Lagerh. f. minor 

 Nordst, in Wittr. & Nordst. ' Alg. Exsic.' 1889, no. 966 ; fasc. 

 21, 1889, p. 45). 



Forma punctata West & G. S. West. (PL L XVIII, 



fig. 9.) 



C. subarctoitm forma punctata "West & G. S. West, Freshw. Alg. Orkneys 

 and Shetlands, 1905, p. 22, t. 2, f . 24. 



Cell-wall distinctly and irregularly punctate. 



Length 16-19 /x; breadth 13'5-lGfi; breadth of 

 isthmus 8'6-10'5//,; thickness 9-10 //,. 



SCOTLAND. Plankton of Loch Beosetter, Bressay, 

 Shetlands ! 



66. Cosmarium pseudarctoum Nordst. 

 (PI. LXVIII, figs. 12-14; PL LXXII, figs. 40, 41.) 



Cosmarium pseudarctoiim Nordst. in Wittr. & Nordst. Alg. Exsic. 1879, 

 no. 257, cum fig. xylogr. ; in fasc. 21, 1889, p. 45 ; Boldt, Desmid. 

 Gronland, 1888, p. 17 ; West, Alg. W. Ireland, 1892, p. 162 ; Alg. Engl. 



