XANTH1DIUM. 87 



spines O'.j-10'5 JJL ; breadth of isthmus 2* 5-3 /A; thick- 



ness 7 {JL. 



KMJIAXD.- -Bog- two miles S. of Clapham, W. Yorks ! 

 Puttenham and Thursley Commons, Surrey ! 



WALES.- -Grlyder Fach, Carnarvonshire ! 



SCOTLAND.- -Rhiconich, Sutherland! Orkneys! Shet- 

 1 a uds ! 



IRELAND. Wicklow (Arcln j r). 



(_7(?<iyr. Distribution.- - Galicia and Bohemia in 

 Austria. Siberia. 



This minute Desmid, first described by Archer as XantJti- 

 <h'ititi concinnum, was afterwards described and figured by 

 Boldt under the name of Arthrodexmus hexagonus. It is 

 characterized bv the hexagonal form of its semicells, by 



*/ O ' 



the single minute spine at each lateral angle, and the pair 

 of similar minute spines at each apical angle. It should 

 be compared most carefully with Cosmarium pygms2um 

 Arch, (vide Vol. Ill, pp. 73-75, t. 71, figs. 22-31). 



Var. Boldtiana West. (PL CXII, figs. 11, 12. 



Arthrodesmus hezaijomis 'forma' Boldt Siber. Chlorophv. 1885, p. 109. t. 5, 



f. 17. 

 Xatithidiittn conclnnnm Arch. var. Boldtimia West, Alg. W. Ireland, 1892, 



p. 167, t, 22, f. 6 ; Eoy & Biss. Scott. Desm. 1893, p. 244 ; W. & G. S. 



West, Alg. S. England. 1897, p. 484 

 Arthrodesmus liexagonus Boldt var. polonica Eichl. & Racib. in Eospraw 



Wydz. matem.-przy. Akad. Uniiej. Krakow, xxvi, 1893, p. 122, t. 3, 



f. b, 7. 



Cells often less deeply constricted ; semicells ellip- 

 tic-hexagonal, without the apical pairs of minute 

 spines. 



Length 10-13 JJL; breadth without spines 10-13*5 /^; 

 breadth of isthmus 2'5-4/x, ; thickness 8-10 /A. 



ENGLAND.- -Tlmrsley Common, Surrey ! 



SCOTLAND.- -Loch Ullachie near Ballater, Aberdeen 

 (Roy # Bissetft. 



IIIELAND.- -Xear Ougiiterard, Galway ! Moher Lough, 

 Mayo ! Lower Lake of Killarney, Kerry ! 



Geofji'. Distribution.- Poland. Siberia. United 

 States. 



This variety differs from typical X. concinnum in the 

 absence of the pairs of minute apical spines. 



