146 BRITISH DESMIDIE.E. 



Tetmemoms Brebissonii differs from T. yramdatus in having its front and 

 lateral views unlike each other, and its puncta arranged in longitudinal lines ; 

 it is also generally smaller. 



Length of immature frond g-i-j of an inch ; breadth ^a'ss '■> breadth at con- 

 striction j^ g'gg ; length of mature frond -^\-^ ; breadth yij ; breadth at con- 

 striction -g-ir* 



Tab. XXIV. fig. 1. a. front view ; b. side view ; e. empty frond ; d, e. front 

 and side views of var. /3 ; f. young frond. 



2. T. lavis (Kiitzing) ; frond in the front view somewhat tapering, with 

 truncate ends ; lateral view fusiform ; puncta none or very indi- 

 stinct. 



Tetmemorus gramdatus (in part), Ralfs, Annals of Natural Hist. v. 14. t. 8. 



f. 2. d, e,f,g (1844) ; Trans, of Bot. Soc. of Edinburgh, v. 2. t. 12. 

 Closterium Iceve, Kiitzing, Phycologia Germanica, p. 132 (1845). 



Dolgelley, J. R. Aberdeen, Dr. Dickie. Ashdown Forest and near Tun- 

 bridge Wells, Sussex, Mr. Jenner. 



Germany, Kiitzing. 



Frond four to six times longer than broad, constricted at the middle, tapering 

 with straight sides ; ends truncate, vdth a hyaline lip, which is inconspicuous, 

 and sometimes absent. 



The puncta are so faint, that even when using the higher powers of Mr. Ross's 

 achromatic microscope I have doubted their existence ; but Mr. Jenner and 

 Mr. Ross assure me that they are scattered, as in Tetmemorus gramdatus. 



I have gathered the sporangia of this species for three successive years near 

 Dolgelley, forming a mucous stratum on the moist soil ; I have also seen them 

 mixed with Desmidiese sent from Aberdeen by Dr. Dickie. 



After coupling, the segments of the fronds are separated by the formation 

 of a large, quadrate, central cell, in which all the contents of both fronds are 

 collected, the empty segments being loosely attached to its corners. The 

 endochrome at first fills the cell, large starch globules being scattered through- 

 out the minutely granular substance ; but at length it becomes a dense, round, 

 homogeneous body of a dark green colour, which finally changes to an olive- 

 brown. In this stage the segments of the original fronds fall off, and leave 

 the quadrate cell inclosing the sporangium. In the front view, as stated above, 

 the cell is nearly square ; the sides are concave, and the angles rounded and 

 slightly produced. A lateral view shows that the cell and sporangium are 

 both compressed. 



In the present plant the process of forming the sporangiiuu is interesting, 

 as it exhibits a striking similarity to the change during the formation of similar 

 bodies in Staurocarpus among the Conjugatte. In Staurocarpus, after con- 

 jugation, a subquadrate cell is formed, within which the endochrome is col- 

 lected. The latter is at first of the same figure as the cell, but in one species 

 at least is at length condensed into a compact globular body, and in every 

 species the cell with the contained sporangium finally separates from the fila- 



