COSMAKIUM. 175 



apex slightly convex. Side view of semicell subcir- 

 cular. Vertical view broadly elliptic, ratio of axes 

 about 1 : 1*2. Cell- wall smooth. 



Zygospore unknown. 



Length 24*5-26 p,; breadth 18-19*5 p.; breadth of 

 isthmus 9 p. 



SCOTLAND.- -Mull in Argyll ; in plankton (Borge). 



C. subaversum Borge should be compared with C. aversum 

 AVest & G. S. West, a species described from Madagascar. 

 It is distinguished from that species by its much broader 

 isthmus, by the more convex apices of the semicells, and by 

 the elliptical vertical view. The cell-wall also is smooth, 

 whereas that of C. aversum is irregularly punctate. 



31. Cosmarium tetrachondrum Lund. 

 (PI. LXII, fig. 1.) 



Cosmarium tetrachondrum Lund. Desm. Suec. 1871, p. 38, t. 3, f . 2 ; Nordst. 

 Xorges Desm. 1873, p. 17 ; Jacobs. Desm. Danem. 1876, p. 195 ; Cooke, 

 Brit. Desm. 1886, p. 89, t. 37, f . 5 ; De Toni, Syll. Alg. 1889, p. 956 ; Eoy 

 & Biss. Scott. Desm. 1894, p. 176 ; Nordst. Index Desm. 1896, p. 253 ; 

 West and G-. S. West, Alg. N. Ireland, 1902, p. 33. 



Ursinella trachondra Kuiitze, Revis. gen. plant. 1891, p. 925. 



Cells small, about 1^ times as broad as long, very 

 deeply constricted, sinus very narrow and linear ; 

 semicells transversely semi-elliptic (or very clepressed- 

 subsemicircular), basal angles rounded, apex truncate- 

 convex, with a granule just above and near the 

 extremity of each linear sinus. Side view of semicell 



e/ 



circular with a basal granule on each side. Vertical 

 view elliptic, ratio of axes about 1 : 2'25, with two 

 granules on each side rather far apart. Cell-wall 

 smooth. Chloroplasts axile, with only one pyrenoid. 



Zygospore globose and smooth, sometimes showing 

 a tendency to elongate irregularly. 



Length 2O5 /^; breadth 23-26*3 p,; breadth of isth- 

 mus 6-7 p.; thickness 1O7 p.. 



ENGLAND.- -Near Chapel Wood, S.E. Surrey ! 



SCOTLAND.- -Ross, Banff, Aberdeen, Kincardine, For- 

 far, and Perth ; near Kingshouse, Argyll ; zygospores 

 from Deeside, Aberdeen (Roy $ Bissett). 



