Hah. Jack’s Wood spring; Cross in Hand, &c.: Mr.
Jenner. Dolgelly and Penzance : Mr. Ralfs.
Tlie fronds are rather large, smooth, deeply constricted in
the centre; the segments, broader than long, are in close approximation
with each other, and when taken together form
an orbicular frond in their front view, in which aspect the
species might be taken for a Cosmarium, from which genus
it differs in the triangular transverse form of the segments.
T. orbicularis forms a distinct link between Cosmarium on
the one side and Desmidium on the other. It is distinguished
from the following species by its suhorbicular frond and by
its blunt angles being destitute of processes.
2. T kigonoctstis mucronata Hass.
Plate L X X X IV . Fig. 8.
Char. Fronds smooth, deeply constricted in the fro n t view, segments
either transversely elliptical or lunate. E nd view
three-lohed. Lobes infiated, blunt, mucronate, d single
mucro terminating each lobe.
a In the fro n t view the segments are transversely elliptic
and the mucro straight.
fl Segments, in fro n t view, transversely lunate, outer margin
straight, mucro curved outwards.
7 Segments, in fro n t view, transversely lunate, outer margin
curved, mucro curved inwards.
Staurastrum mucronatum Ralfs, in Annals, vol. xv. p. 152.
pi. X. figs. 5, 6.; Jenner, in loc. cit. p. 192.
Hah. On Ashdown F o re s t; Piltdown Common ; Fisher’s
Castle, and near Cross in H a n d : Mr. Jenner. Dolgelly
and P enzance: Mr. Ralfs.
This, although a variable species, would appear to be well
characterised by the lobed form of the segments viewed endways
and the presence of the spine-like teeth.
Mr. Ralfs and Mr. Jenner have observed the occurrence
and formation of sporangia in this species, resulting as in other
Desmideæ from the union of the endochromes of two cells,
which in them is never effused from one ceU into another,
but always into a distinct receptacle intermediate between
the two fronds: this is also the case in certain species of
Tyndarideoe and Mesocarpi amongst the Conjugateoe.
The sporangia in T. mucronata are circular and smooth,
and at first enclosed in a membrane larger than itself, this
gradually disappears, and the sporangium becomes spinous,
in which state it bears some resemblance to a Xanthidium.
I have observed occasionally the sporangia of the Vesicur
liferæ to present an appearance exactly similar, a fact which
has not escaped the attention of Mr. Thwaites.
3. T rigonocystis muricata Hass.
Plate L X X X IV . Fig. 9, 10.
Char. Fronds muricated, deeply constricted, transversely elliptic.
Segments approximate. End view triangular, with
convex sides.
fl Yxonds furnished with short spines.
Meneghini, Synops. Desmid. p. 266. Rinatella muricata
Breb., Alg. Fal. p. 66. (1835). Desmidium apiculosum
Ehr., Infus. p. 142.; Ealfs, in Annals, vol. xv. p. 154.
pi. xi. fig. 1.; Jenner, in loc. cit. p. 194. — /3. Xanthidium
deltoideum Corda, Observ. Microscop, sur les Aui-
malc. de Carlsbad, p. 29. pi. v. figs. 38, 39.
Hah. In small pools ; Cheshunt: A . H. H . Weston Bogs,
Hants; Rackham Common, near Pulborough; Piltdown
Common, near U ckfield; Mayfield and Heathfield.
Sussex: Mr. Jenner. Dolgelly and Penzance: Mr.
R a lfs.—fl. Sussex: Mr. Jenner. Dolgelly: Mr. Ralfs.
From Staurastrum tricorne this species may he at once distinguished
by the convex sides of the segments in the end
view, and by the adult fronds being not merely muricated
but spinous. The variety is probably distinct.