HYALOTHECA. 229 



Arch, in Pritch. Inf. 1SG1, p. 722. 



Rabenh. Flor. Europ. Alg. 1808, p. 151. 



Delp. Spec. Desm. subalp. 1873, pp. 23, 47. 



Wood, Freshw. A]g. X. Amer. 1873, p. 124. 



Kirchn. Alg. Schles. 1878, p. 131. 



Wolle, Desm. U. S. 1884, p. 22. 



Cooke, Brit. Desm. 1887, p. 7. 



De Toni, Syll. Alg. 1889, p. 785. 



Wille in Eiigler Xaturl. Pflanzenf. 1890, p. 16. 



Comere, Desm. de France, 1901, p. 196. 



G. S. West, Brit. Freshw. Alg. 1904, p. 176. 



Cells subcylindrical. shorter or longer than broad, 



V O 



very slightly constricted, semicells trapezoid, sub- 

 quadrate or oblong, with straight or slightly convex 

 lateral margins ; cells united by their broadly truncate 

 apices to form long filamentous colonies, which are 

 sometimes twisted and almost invariably enveloped in 

 a gelatinous sheath of some thickness. Chloroplasts 

 axile, one in each semicell. usuallv with a central 



*/ 



pyrenoid, and several radiating ridges. 



Zyuospores globose, smooth, sometimes enclosed in an 

 irregularly shaped structure consisting of the gamet- 

 angia united by the broad conjugation tube. Prior to 

 conjugation the filaments break up into individual cells. 



Aplanospores are known in two species of the genus. 

 They are formed from the ordinarv cells bv the rounding 



/ / */ 



off of the cell contents, which then acquire a strong cell- 

 wall of their own. The formation of aplanospores is 

 also accompanied by the total or partial dissociation of 

 the filament into individual cells. 



The genus Hyalotheca is very closely allied to Spondylosium, 

 from which it is distinguished bv its extremely slight develop- 



j / / 



ment of a sinus, and also by the usually circular outline of its 

 cells in the vertical view. There are 5 British species only, of 

 which one is abundant and universal in its distribution, whilst 

 three are amongst the rarest of British Desmids. 



1. Hyalotheca dissiliens (Sm.) Breb. 

 (PL CLXI, figs. 16-27.) 



Conferva dissiliens Smith, Ens:!. Botairv, v. 35, 1812, t. 2464. 

 Desmidium nutcosum Breb. Alg. Falaise, 1835, p. 65, t. 2 ; Menegh. Synops 

 Desm. 1840, p. 4 ; Ralfs in Ami. Mag. Xat, Hist, 1843, p. 374, t. 8, f. 2. 

 Giceoprium dissiliens Hass. Brit. Freshw. Alg. 1845, p. 346, t. 83, f. 34. 



