Mr. J. Ralfs on the British Desmidiese. 15 



alternate occurrence of two small and two large segments is in- 

 correct, for although the smaller segments are necessarily in pairs, 

 being formed during the elongation and bisection of the cells, I 

 have never seen all the joints thus dividing at the same time, 

 which must always happen if his statement be correct ; on the 

 contrary, many joints with equal segments will often succeed each 

 other, here and there followed by the unequal segments, as I have 

 described above. When the reproductive granules are perfected 

 and the filament of the mature plant ceases to elongate, all the 

 joints will be found similar and their segments equal. 



In my former notice of this plant I described it as destitute of 

 a mucous sheath ; I have not gathered it since ; but on further 

 consideration 1 believe that the sheath exists, although I failed to 

 detect it. The reason for coming to this conclusion is, that the 

 filaments when gathered were remarkably distinct and parallel, 

 exactly as those in Glaoprium mucosum ; I have no doubt that in 

 both this appearance is an effect of the same cause, and that 

 broad colourless sheaths separate the colom'cd filaments. I sup- 

 pose therefore its tenuity, want of colour and great breadth pre- 

 vented its observation. 



Plate III. fig. 7. Spharozosma unidentata : a, portions of filaments ; 

 h, front view of an empty joint ; c, joint with one of its segments newly 

 formed ; d, transverse view. 



2. S. excavata. Joints longer than broad, with a deep excavation 

 on each side, and two sessile glands on each margin at their junc- 

 tion. 



Pools, Dolgelley and Penzance, J. R.; Cross-in- Hand, and Ash- 

 down Forest, Sussex, and bogs at Fisher's Castle, Kent, Mr. Jenner. 



Very minute, seldom more than twenty joints in the filament, 

 which is fragile, and finally separates into single joints ; at their 

 junction, in the front view, are two minute processes or glands 

 invisible before the escape of the endochrome, and situated one 

 near each angle. The joints are from one and a half time to 

 twice as long as broad, much constricted in the middle ; the con- 

 striction is like an excavation or broad sinus on each side, so that 

 the margins of the filament appear sinuated. The transverse view 

 is oblong with four sessile minute glands, two on each side and 

 situated near their ends. 



The endochrome is pale bluish green with minute scattered 

 granules. 



Plate III. fig. 8. Sphcsrozosma excavata: a, portion of a filament; b, 

 front view of an empty joint; c, transverse view. 



