curious, and is confined to this and the following species.
The design of the interrupted conjugation is by no means
evident: it is remarkable, however, that the granules in those
cells, which remain disunited, decrease in size and become
almost colourless, Avhile those of the conjoined cells increase
considerably, and darken in colour. This mode of conjugation
at least affords a decisive character by which it and th»
following species may be known from all others hitherto described.
One fact is still Avanting to complete the history of
this species, viz. the form of the sporangia, which might
possibly furnish an additional distinctive character.
3. Zygnema interruptum Hass.
P late X X L .
Char. Fdaments o f considerable length, but less in diameter
than those o f Z. orbiculare. Cells at the period o f conjugation
rather longer than broad: previous to this, however,
theij are frequently not h a lf so long as broad. Spires numerous.
Conjugation interrupted. Spire oval, equalling in
breadth the diameter o f the cell but not producing any
inflation o f it.
Hassall, in Annals of Nat. Hist. vol. xi. p. 432.
Hab. Cheshunt: A. H. H.
This also is a very rare species. I have only met with It
once, but then in very considerable quantity. I t bears a
close resemblance to Z. alternatum, but differs from it in
having filaments which are considerably smaller than those in
that species. The granules or unfertilized zoospores are likewise
largest in those cells which have conjoined.
4. Zygnema serratum Hass.
Plate X X I I I . Figs. 1, 2.
Char. Filaments o f nearly the same diameter as those o f
Z. orbiculare, but less mucous. Cells longer than broad.
Spires varrjing from three to fiv e in number. Granules
larger. Conjugation continuous. Sporangia broadly
ovate.
Hassall, in Annals of Nat. Hist. vol. xi. p. 432.
Hah. Cheshunt, very ra r e : A. H. H. Withy ham : Mr.
Jenner.
The only species with which this can possibly be confounded
is Z. orbiculare, from Avhlch it may be readily distinguished by
the fewer number and serrated appearance of the spires, the
larger size of the granules, and the form of the sporangia,
which in Z. orbiculare are nearly spherical, and compressed,
while In Z . serratum they are broadly ovate. The filaments
do indeed resemble very closely in character those of Z . nitidum,
Avith which, however, there is but little danger of confounding
it, that species being altogether a much smaller plant.
It is very rare.
5. Zygnema nitidum Ag.
Plate X X I I . Figs. 1, 2.
Char. Filaments qC much less diameter than those o f Z. serratum.
Cells usually rather more than twice as long as hroad.
Spires generally fo u r in number. Granules large. Sporangia
acutely ovate.
Conferva jugalis Müller, Flora Danica. Conjugata princeps
Vaucher, Hist, des Conf. d’F au douce, p. 64. pi. iv. fig. 1.
F. nitidum Harv. in Manual, p. 143. ; Agardh’s Systema.
Conf. nitidum DilLvyn, British Confervoe, pi. iv. f. c. ;
Fng. Bot. t. 1656, In part.
Hab. F very where common throughout Great Britain
and Ireland.
This species is one of the best marked as well as most
beautiful of the tribe. Numerous other Zygnemata have
hoAvever doubtless been associated with it under the same
name.
Vaucher has the following observation upon it : — “ Independently
of its size, it is distinguished from all others by a
coarser touch, a more shining appearance of tubes almost crisp,
and by its constant habit of raising its extremities out of
the water whenever it is immersed in the liquid.”