Mr. J. Ralfs on the British Desmidiez. 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 jomts 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 sueceed 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 thew segments equal. 
In my former notice of this plant 1 described it as destitute of 
a mucous sheath; I have not gathered it since; but on further 
consideration I 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 m Gleoprium mucosum; | have no doubt that in 
both this appearance is an effect of the same cause, and that 
broad colourless sheaths separate the coloured filaments. I sup- 
pose therefore its tenuity, want of colour and great breadth pre- 
vented its observation. 
Prare III. fig. 7. Spherozosma unidentata: a, portions of filaments ; 
b, 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 jomts im 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, 
Prarte III. fig. 8. Spherozosma excavata: a, portion of a filament; 0, 
front view of an empty joint; ¢, transverse view. 
