BRITISH DESMIDIEiE. 61 



Traces of a mucous sheath have been detected by the Rev. M. J. 

 Berkeley and more recently by myself. 



A transverse view shows that the cell is triangular or quadrangular, 

 and that the endochrome has thick rays corresponding in number 

 with the angles ; these rays are frequently cloven . 



Recent specimens of Desmidium are easily known from other 

 genera by one or two dark waved lines passing down the filament, 

 which appearance is occasioned by the twisting of the angular 

 filament. 



1. D. Swartzii (Ag.) ; filament triangular, equal, with a single longi- 

 tudinal, waved, dark line formed by the third angle ; end view 

 triangular with the endochrome three-rayed. 



Diatoma Swartzii, Ag. Disp. Alg. Svec. (1811). Lyngb. p. 1/7. t. 61. 



Desmidium Swartzii, Ag. Sijst. p. 9 (1824). Consp. Crit. Diat. p. 56. Gre- 

 ville, Scot. Cryp. Fl. t. 292. Kiitzing, Synopsis Diatom, in Linncea 

 1833, p. 613 ; Phycologia Germ. p. 141. Greville in Hook. Bi-it. Fl. 

 V. 2. p. 402. Bre'bisson, Alg. Fal. p. 53. t. 2. Ehrenberg, Infus. 

 p. 140. t. 10. f. 8. Meneghini, Synop. Desmid. in Linncea 1840, p. 203. 

 Bailey, American BaciUaria in Amer. Journal of Science and Arts, v. 41. 

 p. 288. t. 1. f. 1. Harvey, Manual of Brit. Algce, p. 196. Corda, 

 Observ. Microscop. sur les Animal, de Carlsbad, p. 17. Ralfs, Annals 

 of Nat. Hist. V. 11. p. 375. t. 8. f. 3 ; Trans, of Bat. Soc. of Edin- 

 burgh, V. 2. p. 7. t. 2. Hassall, Brit. Freshwater Algce, p. 344. t. 83. 

 f. 7. 



Common. Appiii, Capt. Carmichael. Timbridge Wells, Mr. Borrer. 

 Swansea ; Carnarvon ; Dolgelley ; Penzance, &c., J. R. Many stations in 

 Sussex, and near Southampton, Mr. Jenner. Essex, Mr. Hassall. Caragh 

 Lake, Kerry, Mr. Andrews. Meath, Mr. Moore. Westmoreland, Mr. Side- 

 botham. Rochdale, Mr. Coates. Manchester, Mr. Williamson. Aberdeen- 

 sliire. Dr. Dickie and Mr. P. Grant. 



Sweden, Agardh. North Germany, Martens, Kutzing and Ehrenberg. 

 Carlsbad ; Prague ; Reichenberg, Corda. Throughout the United States, 

 Bailey. Falaise, Brebisson. 



The filaments are elongated and very fragile, but separate spontaneously 

 into single joints less readily than in Hyalotheca dissiliens and Didymoprium 

 Grevillii. They adhere but slightly to paper. They are triangular, and, 

 when viewed under the lens, the margins are always formed by two of the 

 three angles in tiirn, whilst the dark line regularly passing from side to side 

 marks the third angle and shows that the filament has a spiral twisting in 

 about 16 joints. On examining the filament where the dark line touches the 

 margin, if we first raise and then depress the lens, both angles at the point of 

 apparent contact may be distinctly seen. 



The joints are in the front view somewhat quadrangular, broader than long, 

 and each angle has two minute, slightly angular teeth. The notch between 

 the teeth is very distinct but not gaping. The joints are connected by a 



