18 BEITISH DESMIDIACE^. 



231. Cosmarium margaritatum (Lund.) Roy & Biss. 



(PL XCIX, figs. 8, 10.) 



Cosmarium latum Breb. var. margaritatum Lund. Desm. Suec. 1871, p. 

 26 ; Nordst. Norges Desm. 1873, p. 12 ; Boldt, Desmid. Gronland, 1888, 

 p. 26 ; De Toni, Syll. Alg. 1889, p. 999. 



C. margaritatum (Lund.) Roy & Biss. Jap. Desm. 1886, p. 194; West, 

 Freshw. Alg. Maine II, 1891, t. 315, f . 16 ; Roy & Biss. Scott, Desm. 

 1894; p. 167, t. 2, f. 12 ; Johnson, Rare Desm/U. S. II, 1895, p. 292, 

 t. 240, f . 32 ; Nordst. Index Desm. 1896, p. 165 ; W. & G. S. West, 

 Desm. Singapore, 1897, p. 165 ; Alg. S. England, 1 897, p. 488 ; Schmidle, 

 Lappmark Siisswasseralgen, 1898, p. 35 ; G. S. West, Alga-fl. Cainbr. 

 1899, p. 217; W. & G. S. West, Alg. N. Ireland, 1902, p. 37; Freshw. 

 Alg. Ceylon, 1902, p. 169 ; Freshw. Alg. Orkneys and Shetlands, 1905, 

 p. 21 ; G. S. West, Alg. Third Tanganyika Expedit. 1907, p. 122. 



Cells fairly large, about 1^ times as long as broad, 

 deeply constricted, sinus narrowly linear with a dilated 

 extremity ; semicells subrectangular (sometimes ellip- 

 soid-subrectangular), apex very slightly convex, sides 

 slightly convex, never upwardly divergent, basal angles 

 rounded, superior angles broadly rounded ; cell-wall 

 uniformly granulate, from 28 to 32 granules showing 

 round the margin of a semicell ; granules solid, dis- 

 posed in oblique decussating series, about 12 series in 

 each direction, and also in indistinct vertical series, 

 cell -wall between the granules punctate, the punctula- 

 tions arranged in hexagons around each granule. Side 

 view of semicell subcircular. Vertical view oblong- 

 elliptic with convex sides. Chloroplasts axile, two 

 pyrenoids in each semicell. 



Zygospore unknown. 



Length 66-1 05 p.; breadth 56-82 //, ; breadth of 

 isthmus 19-31 p. ; thickness 40-48 /A. 



ENGLAND.- -Austwick Moss, W. Yorks ! Chippenham 

 Fen, Cambridge ! Frensham Great Pond and Thursley 

 Common (very abundant) , Surrey ! New Forest, Hants ! 



WALES.- -Llyn-y-cwm-ifynon and Capel Curig, Car- 

 narvonshire ! Plankton of several of Welsh Lakes ! 



SCOTLAND. Pool near View Rock, Strathpeffer, 

 Ross ; Slewdrum, Aboyne, Cambus O'May, Dalbagie, 

 Grlen Clunie, and Castleton, Aberdeen (Roy fy Bixsett). 

 Plankton of Loch Doon, Ayr ! Rhiconich, Sutherland ! 



