12 Mr. J. Ralfs on the British Desmidiez. 
mon. Most plentiful in the autumn. Bantry, Miss Hutchins ; Appin, 
Capt. Carmichael. Trentishoe, Devonshire ; Penzance and Dolgelley, 
J.R.; Cheshunt, Mr. Hassall; Chiltington Common near Pulbo- 
rough, Sussex; and in the peat bog at Fisher’s Castle near Tun- 
bridge Wells, Mr. Jenner. 
Filaments elongated, very gelatinous, of a pale translucent 
green, not fragile. Under the microscope the joints are generally 
about equal in length and breadth, and the endochrome forms a 
single irregular patch. The joints are not constricted, but at one 
end they have on each margin a minute bidentate projection: as 
the similar ones of the next joint are at its adjoming extremity, 
these projections oecur only near the alternate dissepiments. 
When, however, the joint is elongated, preparatory to the forma- 
tion of two joints, the endochrome is divided into two portions, 
and then these processes are present at both ends, the next joint 
undergoing a similar change. These projections are extremely 
minute, and can only be detected by employing the highest power 
of the microscope, and even then are liable to be overlooked if 
not carefully sought for. 
I examined many specimens of this plant in 1841, but did not 
perceive these curious projections until the followmg year. I 
believe they are really formed by a grooved rim round the end of 
the joint, because however the tilament may be moved they are 
equally apparent ; whereas if they were processes, as in Didymo- 
prium, they would be sometimes either entirely concealed or ren- 
dered less apparent. 
The filaments have a very broad mucous sheath, which from 
its great breadth and absence of colour is not easily discerned ; 
it is more evident when a specimen is dried on talc or glass, as 
the margins are then generally perceptible. When gathered the 
filaments are very distinct, frequently parallel and subdistant 
even to the naked eye : this depends on the great breadth of their 
mucous sheaths, which prevent the coloured filaments coming into 
contact. By this character G. mucosum may in general be known 
even without the aid of a microscope. The same circumstance 
occwrs In young plants of G. dissiliens, but is less remarkable, as 
its mucous sheath is not more than half as broad. 
Under a low power of the microscope G. mucosum has consi- 
derable resemblance to G. dissiliens, with which it is probably not 
unfrequently confounded, But they may always be distmguished 
even without the aid of the microscope. The G. dissiliens is ex- 
tremely fragile, and will break into pieces if a small portion be 
placed on the hand and the finger gently passed over it; this plant, 
on the contrary, will not break if it be taken out of the water and 
allowed to hang down in long strings. It always has a clear 
translucent appearance ; the G, dissiliens, except when very young, 
