54 BRITISH DESMIDIE.^i:. 



Ill shallow pools and gently-flowing streams ; most {)Ientiful in the autumn. 

 Gantry, 3Iiss Hn f chins. Appin, Cupt. Carmichael. .Trentishoe, Devonshire ; 

 Penzance ; and Dolgelley, J. R. Herts and Essex, Mr. HassctU. Chiltington 

 Common, near Pulborough, Sussex ; Ashdown Forest ; and in the peat-bog at 

 Fisher's Castle, near Tunbridge Wells, JSL'. Jenner. Near Aberdeen, Mr. P. 

 Grant. 



Falaise, France ; Brebisson. 



Filaments elongated, very gelatinous, of a pale translucent green ; not fra- 

 gile. 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 adjoining extremity, 

 these projections occur only near the alternate dissepiments. When however 

 the joint is elongated, preparatory to the formation of two joints, the endo- 

 chrome is divided into two portions, and then these projections are present at 

 both ends, the next joint undergoing a similar change. These projections are 

 extremely minute, and can be detected only by employing the highest powers 

 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 protuberances until the following year. I believe they 

 are really formed by a grooved rim round the end of the joint, because how- 

 ever the filaments may be moved, they are equally apparent ; whereas if they 

 were real processes, as in Didymoprium, they would be sometimes either en- 

 tirely concealed or rendered less distinct ; they can also be traced like a trans- 

 verse band across the empty cell. 



The filaments have a very broad mucous sheath, which from its gi'eat 

 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 percep- 

 tible. Wlien first 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 prevents the coloured filaments from coming into 

 contact. By this character H. mucosa may in general be known, even without 

 the aid of a microscope. The same circumstance occurs in young plants of 

 H. dissiliens, but is less remarkable, as its mucous sheath is not more than 

 half as broad. 



Under a low power of the microscope, H. mucosa has considerable resem- 

 blance to H. dissiliens, vdth which it is probably not unfrequently confounded. 

 But they may always be distinguished, even without the aid of a microscope. 

 //. dissiliens is extremely 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 

 dowi^ in long strings. It always has a clear translucent appearance ; but H. 

 dissiliens, except when very young, is of an opake green. H. dissiliens, if 

 kept in water for a few days, spontaneously separates into single joints ; //. 

 mucosa, although treated in the same manner for weeks, did not separate into 

 fragments sufficiently small to enable me to obtam a satisfactory transverse 

 view. At length, however, I was more fortunate. When kept in water the 



