b. Filaments triangular.
* Mucous sheath not apparent.
3. D esmidium Swaktzii Ag.
Plate L X X X I I I . Figs. 7, 8.
Char. Filaments triangular. Cells rather shorter than hroad,
hicrenate, angles o f the cells well marhed. Endochrome
three-raged.
Grev. Scot. Flor. t. 292.; Ehrcn. Die Infus. p. 140. t. 10.
f. 8.; Pritch. Infus. p. 183.; Harv. in Manual, p. 196.;
Ag. Syst. p. 9 .; Ralfs, in Annals of Nat. Hist. vol. xi,
p. 375. plate viii. fig. 3 .; Jenner, in loc. cit. p. 192.
llah. Scotland : Dr. Greville. Appin : Captain Cai'-
michael. Tunbridge Wells: 31r. Borrer. Swansea,
South Wales ; Caernarvon and Dolgelly, North Wales;
Penzance: Mr. Ralfs. High Beech, E p p in g : A. H. H.
Rusthall Common, and in a pond between Ramslye
rocks and Broadwater Forest; Battle; Chiltington Common,
near Pulborough: Mr. Jenner. Bog holes, co.
Meath: Mr. 3Ioore.
In consequence of the filaments of this species being triangular,
the spiral twisting of the threads is more evident than
in any other species of the genus. When the microscope
is brought to bear upon any filament, two of the three lines
of crenatures are visible, and the third is indicated by a dark
waved line passing from one margin of the filament gradually
towards the other, the dark appearance of the line being
occasioned by the dense endochrome of the cells situated
immediately behind the crenatures.
Kützing limits the genus Desmidium to this one species,
which he considers to difier generically from the other species
usually associated with that genus, in the fact of the presence
of the third angle to the cells — a distinction not considered to
to be sufiicient in the case of the genus Staurastrum for
the foundation of new genera. The German professor has
therefore been induced to institute a new genus for D.
cglindricum, under the name of Didymoprium. This genus,
in my humble opinion, ought not to be adopted ; and if it be
reaUy a good and valid genus, then is there sufiicient reason
why D. Borreri should form the type of a genus different
from both, for it differs from D. Swartzii in the cells being
biangular and not triangular, and from Desmidium cylindricum
Crev., the Didymoprium Grevillei of Kützing, in the absence
of a mucous sheath. There would thus be as many genera as
there were formerly considered to be species of Desmidia, a
result not altogether satisfactory.
c. Filaments with fo u r angles.
4. D esmidium quadkangulatum Ralfs.
Plate L X X X IV . Fig. 3.
Char. Filaments quadrangular. Endochrome/owr-rayeiZ.
Ralfs, in Annals, vol. xv. p. 405. pi. xii. fig. 9.
Ildb. In a boggy pool at Balogas, near Penzance: Mr.
Ralfs.
Notwithstanding the strong resemblance which it bears to
Desmidium Swartzii, this plant appears to me to be very distinct.
In consequence of the filaments having four angles
instead of three, and as they are spirally twisted in the
same manner as those of other Desmidia, it follows, that two
dark, and waved lines, describing two of the four angles, are
visible in the length of the thread. The observation of these
will at once serve to distinguish it from Desmidium Swartzii,
of which Mr. Berkeley, Mr. Borrer, and Mr. Ralfs consider
this plant to be a variety: the latter, however, remarks
tha t he has gathered it for two successive years quite unmixed
with that species.
I t seems to me that this plant has not merely a right to be
considered as specifically distinct from D. Swartzii, but nearly
as much claim (a claim however which I do not allow) to be