Mr. J. Ralfs on the British Desmidiese. 11 



with either a slight constriction which produces a crenate ap- 

 pearance, or a grooved rim at one end which forms a bifid pro- 

 jection on each side. 



The filaments are cylindrical, simple, jointed, invested with a 

 broad gelatinous sheath, and very fragile in one species, but not 

 so in the other. Either a groove passes round each joint, giving 

 a crenate appearance to the margins of the filament, and dividing 

 the endochrome into two portions, or else a grooved rim at one 

 extremity of the joint appears on each side like a bifid process. 



A transverse view shows a radiate endochrome in one species, 

 but in the other I was unable to obtain this view on account of 

 its want of fragility. 



The cylindrical filaments distinguish this genus from Desmi- 

 dium and Spharozosma. From Didymoprium it diff*ers in the 

 absence of angular projections, in not being twisted, and in al- 

 ways having the same apparent breadth. 



1 . G. dissiliens. Filaments fragile, crenate ; a shallow groove round 

 each joint divides the endochrome into two portions. Desmidium 

 viucosutn, Breb. Alg. Fal. p. 65. pi. 11 ; Menegh. Synop. Desmid. 

 in Linnsea 1840, p. 204; Ralfs in Annals of Nat. Hist. vol. xi. 

 p. 374. pi. 8. fig. 2. 



This plant is apparently common, as, in addition to the habitats 

 already given, Mr. Jenner has gathered it in numerous stations 

 both in Sussex and in Kent. It has also been gathered in Ire- 

 land, near Bandon, by Dr. Allman, and in Kerry by Mr. Andrews. 



In an advanced state it becomes of a pale opake green. 



The mucous sheath is easily perceived, and is on each side of 

 the filament as broad as the central coloured portion. The en- 

 dochrome is divided into two portions by the central constriction, 

 which can always be detected on a careful examination with the 

 higher powers of the microscope. 



This plant has been involved in much confusion ; it was by 

 mistake figured in ^Eng. Bot.' for the Conferva dissiliens of 

 Dillwyn, and afterwards altogether omitted in Hooker^s ^Br. 

 Flora ^ and in Harvey^s ^ Manual of the British Algse.^ From 

 the synonyms in Meneghini^s ' Synopsis Desmidiearum,^ it seems 

 that De Brebisson considered it the Conferva mucosa of Mertens 

 and Dillwyn. 



2. G. mucosum. Filaments scarcely fragile ; joints not constricted, 

 but having at one of the ends a minute bidentate projection on 

 each margin, the adjoining end of the next joint bearing similar 

 projections. Conf. mucosa, Mert. ; Dillw. Brit.Conf. tab. B. ; Hook. 

 Br. Fl. vol. xi. p. 351 ; Harv. Br. Alg. p. 127. Gloeotila monili- 

 formis, Kutz. Phyc. Generalis, p. 245 ? 



In shallow pools and gently-flowing streams, probably not uncom- 



