18 BRITISH DESMIDIACE^. 



dilated apex ; semicells subquadrate, slightly narrowed 

 from base to apex, sides (including the angles) 4-undu- 

 late, upper and lower angles slightly rounded, apex 

 with two undulations. Side view of semicell sub- 

 elliptic. Vertical view elliptic, ratio of axes about 

 1 : 1*6. Cell-wall smooth. One axile chloroplast in 

 each semicell containing a central pyrenoid. 



Zy go spore unknown. 



Length 35-45 //,; breadth 21*5-25 p,; breadth of 

 isthmus 9-1 2 ju,; thickness 1618^. 



ENGLAND.- -Bowness, Westmoreland (BisseH). Near 

 Ilkley and Penyghent, W. Yorks ! Mickle Fell, 

 N. Yorks ! Epping Forest, Essex ! 



WALES.- -Bettwys-y-Coed and Llyn Ogwen, Car- 

 narvonshire ! 



SCOTLAND. Corrie Kandor, Aberdeen ! 



IRELAND. Dublin and Wicklow (Archer). 



Geogr. Distribution.- -France. Germany. Hungary. 

 G-alicia. Italy. Norway. Sweden and Bornholm. 

 Poland. Russian Lapland. N. Russia (var.). Green- 

 land. Siberia (var.). 



C. tetragonum is a rare species occurring in similar situa- 

 tions to those which yield its nearest ally, C. notabile Breb. 

 It differs from the latter chiefly in its more quadrate and less 

 attenuated semicells, and in its narrower and usually deeper 

 sinus. We have not examined any specimens which corre- 

 spond exactly with Nageli's figures, the upper angles of the 

 semicells being invariably more prominent. 



Some of the varieties of this species are more general than 

 the typical form. 



Var. Lundellii Cooke. (PL LXVI, figs. 23, 24.) 



C. tetragonum forma Lund. Desm. Suec. 1871, p. 42, t. 2, f. 21 ; Arch, in 



Quart. Joiirn. Micr. Sci. 1877, p. 102 ; Roy & Biss. Scott. Desm. 1894, 



p. 176. 

 C. tetragonum var. Lundellii Cooke, Brit. Desm. 1887, p. 98, t. 37, f. 18; 



West, Alg. N. Wales, 1890, p. 289 ; Heimerl, Desm. alp. 1891, p. 598 ; 



West, Alg. Engl. Lake Distr. 1892, p. 724 ; Liitkem. Desm. Attersees, 



1893, p. 550 ; Johnson, Eare Desm. U.S. II, 1893, p. 294, t, 240, f . 21 ; 



West & G-. S. West, Alga-fl. Yorks. 1900, p. 81 ; Freshw. Alg. Orkneys 



and Shetlands, 1905, p. 21. 

 ? C. bigorrense Gray, Alg. Bagn. Bigorr. 1891, p. xxxi (cum fig.). 



Semicells with the sides somewhat more convex and 



