DUBLIN UNIVERSITY ZOOLOGICAL AND BOTANICAL ASSOCIATION. 247 
even in many of those of more simple, even cylindrical outline, there is 
often a difference of colour in the cell-wall marking the newly grown 
portion. For example, Penium cylindrus, in which the cell-wall of the 
older segments being reddish, the newly grown portion is well marked 
by its absence of colour. So also in Closterium and others. But in the 
form under consideration, the sides being parallel and straight, and the 
cell-wall destitute of colour, Ido not see any external means of proving 
that the new growth of each joint takes place only between the older 
segments. In the bipartite forms (such as Micrasterias, Euastrum, Stau- 
rastrum, &c.), the mode of increase is necessary in order to restore the 
symmetry of the dividing frond; but the form in question being desti¬ 
tute of lobes, inflations, or processes, and straight, the normal symmetry 
of the cylindrical dividing joint is not disturbed. Nevertheless (while 
it seems to me that the characters before detailed are abundantly suffi¬ 
cient, in fact do prove, that this organism is a true Desmid, for it will 
be noticed that the mode of cell-increase referred to does not form part 
of Balls’ diagnosis of the Desmidian group), though it cannot be de¬ 
duced, from any alteration in the outivard form of the joints, that the 
new growth takes place in the manner described, yet I think the fol¬ 
lowing circumstance indicates sufficiently definitely that this is really 
the mode which holds good, and to my mind it leaves no doubt. 
On looking over a mass of the filaments upon a slide, in by far the 
greater number of the joints it may be seen that the pale interruption 
of the endochrome occupies the centre of the joint, and is distant from 
each extremity precisely, or very nearly precisely, the same interval. 
But in a few cases, it may sometimes be noticed, that in two neighbouring 
joints of a filament the pale space is not central, but in both is very con¬ 
siderably nearer the adjacent extremities of the two contiguous joints. 
It will also be remarked that the two joints displaying this peculiarity 
are also shorter than the remaining joints of the same filament: Eig. 3, 
representing a few joints of a filament (magnified 200 diameters), shows 
at its upper part the state alluded to. Now I believe that this occurs 
in the following way. A septum is formed—as in the other Desmidians 
under the isthmus at the constriction—here, at the pale space in the 
centre of the cylindrical joint, which represents the isthmus in the bi¬ 
partite forms. Close to this new septum, at each side, there now 
exists an interruption of the cell-contents forming the pale space, 
which, by the growth (perhaps pretty rapidly) of the interval between 
it and the lately formed septum, and by a fresh accession of endochrome, 
soon, in place of being, as hitherto, eccentric, becomes removed to its 
normal or regular central position; and this taking place in each joint, 
the equilibrium of both is restored, because, moreover, the segments 
remote from the new septum, in both cases, seem to remain un¬ 
altered. This is as precisely similar to the mode of increase which is 
usually seen, here and there, in a few of the joints in the filaments of 
Sphcerozosma vertebratum, as is compatible with the great difference of 
the form of the joints in each species. In Spheerozosma vertebratum it is 
more apparent, because the constricted form of the joints renders it in- 
