58 BRITISH DESMIDIACEJL. 



semicells, being strictly analogous to X. antilopseum var. 

 triquetrum Lundell, and as in that variety the thickened central 

 area is present in the middle of each of the three sides. It 

 is at its greatest abundance from July to September. 



3. Xanthidium tetracentrotum Wolle. 

 (PL CVII, figs. 1, 2.) 



Xanthidium tetracentrotum Wolle in Bull. Torr. Bot. Club, 1882, p. 29, t, 

 13, f. 14: Wolle, Desm, U. S. 1884, p. 95, t. 22, f. 8-9; De Toni, Syll. 

 Alg. 1889, p. 923; W. & G. S. West, Some N. Amer. Desm. 1896, 

 p. 253, t. 15, f . 24- ; Cushman in Rhodora, vii, 1905, p. 260, t. 64, f. 8, 9. 



Arthrodesmus incrassatus Lagerh. Bidrag. Amerik. Desm.-fl. 1885, p. 242, 

 t. 27, f. 18; Wolle, Freshw. Alg. U. S. 1887, p. 35, t. 61, f. 6; De Toni, 

 Syll. Alg. 1889, p. 1061. 



Cells somewhat small, a little longer than broad, 

 deeply constricted, sinus widely open outwards with 

 a submamillate extremity ; semicells subelliptic or 

 reniform-elliptic, with the dorsal margin more convex 

 than the ventral margin, slightly flattened at the 

 middle of the ventral side and with the cell-wall 

 faintly thickened in the middle of the apex, each 

 lateral angle furnished with either one or two simple, 

 elongated, outwardly-curved spines ; in the centre of 

 the semicell with a thickened (and usually yellow- 

 brown) area of considerable size, possessing prominent 

 scrobiculations which are variable in size, number, and 

 disposition. Side view of semicell subcircuiar or 

 obovate-subcircular, with a conspicuous thickening of 

 the wall on each side. Vertical view rhomboid-elliptic, 

 with either one or two simple spines at each pole, and 

 a prominent thickening of the wall in the median part 

 of each side. 



Zygospore unknown. 



Length 41-50 ju; breadth without spines 33-40 /x, 

 with spines, 54-66 /x; breadth of isthmus 12 /x ; thick- 

 ness 23-25 /x. 



Geogr. Distribution.- -United States. 



The typical form of this species is not known to occur in the 

 British Islands. Its most characteristic feature is the large 

 thickened and scrobiculated area in the centre of the semi- 

 cell. The single or paired lateral spines are also distinctive. 



