GG BRITISH DESMIDIE.E. 



Sehistochilum unidejitutum, Ralfs, in Jenrn^ra Fl. of TunhriJge Wells, p. 192 



(1845). 

 Sphterozostna unidentatvm, Ralfs, in Annals of Nat. Hist. v. 16. p. 14. t. 3. 



f. 7. (1845) ; Trans, of Bot. Society of Edinburgh, v. 2. p. 167. 

 Isthnosira vertebrata, Kiitz. Phycologia Germanica, p. 141 (1845). 



Hab. Chy-au-hal and Kerris Moor near Penzance ; Dolgelley, /. R. Ro- 

 therfield and near Tunbridge Wells, Mr. Jenner. Isle of Sheppey, Mr. 

 Bowerbank. Ambleside, Mr. Sidebotham. 



France, De Brebisson. Carlsbad, Corda. North Germany, Eh'enberg and 

 Kiitzing. West Point, New York, Bailey. 



The filaments are much compressed, and the joints, which are nearly equal 

 in length and breadth, are so deeply constricted that at first sight a single 

 one might be mistaken for two. This is more especially the case whilst the 

 endochrome obscures the view of the union of the segments ; as soon however 

 as the joint becomes empty, its nature is distinctly seen. Though in other 

 respects symmetrical, one segment is frequently much smaller than the other. 



At the centre, where the joints are connected, is a minute gland or process 

 arising from each margin. The projection of these glands is easily seen, even 

 before the joints separate. 



By a transverse \'iew the joints are shown to be compressed, and the oblique 

 glands, which are globular and supported on a very short stipes, are very 

 distinct. In this view the joints are about twice as long as broad, with slightly 

 concave sides, and the endochrome is stellate with from four to six rays. 



In a recent state Sphcerozosma vertebratum is very gelatinous and furnished 

 with a broad mucous sheath, which from its tenuity and want of colour is 

 very difficult of detection, and consequently I at first described it as absent. 

 When the plant occurs unmixed with others, the presence of the sheath is 

 indicated by the distinct and parallel filaments exactly as in Hyalotheca 

 mucosa ; for in both plants this appearance is produced by its great breadth, 

 which prevents the closer contact of the coloured centres. The sheath is 

 sufficiently apparent in specimens preserved on glass, and is on each side 

 nearly as broad as the filament. In dried specimens it has an irregular waved 

 margin and faint transverse markings. 



The greater difficulty of detecting the sheath in recent specimens of Hya- 

 lotheca mucosa and S. vertebratum than in Hyalotheca dissiliens and Didy- 

 moprium Grevillii, seems to depend more upon its condition in the two former 

 than upon its greater breadth. As in them it is less condensed, it is not only 

 liable to be confounded with the water in which it is viewed, but is also more 

 soluble in it. When Didymoprium Grevillii is mounted in fluid, the sheath 

 remains as distinct as in recent specimens ; the case is nearly the same when 

 Hyalotheca dissiliens is so treated, but not so in respect of the present plant, 

 the sheath of which seems to be dissolved. 



Length of joint xtt? ^^ ^^ ^^^ ' breadth of segments from ^^ to -^ ; 

 breadth at constriction -^rrr 5 diameter of sporangium y^Vo • 



Tab. YI. f. \. a, b. portions of filaments; c. empty joints; d. side view; 

 e. glands in front ^iew ; /, g. transverse views. Tab. XXXII. f. 2. a, b, c. 

 sporangia. 



