BRITISH DESMIDIEiE. 159 



I am doubtful whether this plant be a true Docid'nim, for in all the other 

 species of this genus there is, at the middle, an evident constriction, and a 

 conspicuous suture where the segments eventually separate ; on the other 

 hand, this plant has neither constriction nor suture. 



Specimens of Doc'uUum asperum sent me by M. de Brebisson are attenuated 

 towards the extremities, whilst the apex itself is dilated. British specimens 

 are not constricted beneath the dilated ends. Brebisson finds moving granules 

 at the extremities as in the other species, but I have not noticed them. 



This plant is not unlike the separated joints of a species of Mougeotia, but 



the dilated ends and roughness produced by minute granules distinguish it. 



Length of frond from gL- to ^ of an inch ; breadth -j-gW '■> breadth at end 

 ] 



2 2T2"- 



Tab. XXVI. fig. 6. a. frond with endochrome ; b. empty frond ; c. empty 

 frond of French specimen. 



16. CLOSTERIUM, Nitzsch. 

 Frond simple, elongated, attenuated, Innately curved or arcuate, 

 entire ; the junction of the segments marked by a pale transverse 

 band. 



In Closterimn the frond is simple, much elongated^ fusiform, rarely 

 cylindrical or prismatic, more or less attenuated at the extremities. 

 It is semilunate, crescent-shaped, or, in a few instances, curved only 

 at the ends, but in the usual position the opposite margins are alvvavs 

 more or less dissimilar, the upper one being convex, and the lower, 

 inclusive of the ends, straight, or more commonly concave ; the lateral 

 view, which differs from the front one, is straight and has both mar- 

 gins similar. As happens in the allied genera, the frond is composed 

 of two segments which finally separate and allow the escape of the 

 endochrome ; the suture or junction is marked by a transverse hyaline 

 or pale band, accompanied in some species by one or more transverse 

 striag, best seen in the empty frond; but there is no constriction. 

 The segm.ents are entire, without spines, processes, or inflated pro- 

 tuberances. 



The endochrome is green with darker longitudinal bands or fillets? 

 the number of which varies in different species. Meneghini considers 

 them of too much importance to be omitted in the specific definition*. 

 They may occasionally be useful in discriminating nearly allied forms, 



* " Interanea initio uniforma serius in tsenias distribuuntur, quarum cha- 

 racteres constantes videntur et in specifica definitione omitti nequeunt." — 

 Menegh. Si/noj). Desmid. p. 230. 



