wm-
2 . M i c r a s t e r i a s r a d i a t a Hass.
Plate XC. Fig. 2.
Ckar. Frond orbicular, smooth. Segments hemispherical
each five-lobed. Lobes approximate, deeply hifid. Divisions
divergent, linear, and slightlg hidentate.
Alicrasterias melitensis Ralfs, in Annals, vol. xiv. p. 260.
pi. vi. f. 2.
Hah. Amongst Diatoma fenestratum in a small pool a
little below the outlet of L lyn Cenernon, near Dolgelly,
very rai-e : Mr. Ralfs.
Tliis species is described by Mr. Ralfs under the name o f
Alicrasferias melitensis, and as the Euastrum crux melitensis
of Ehrenherg, which species it seems to me th a t it assuredly
is not. In the true Alicrasterias crux melitensis the
frond is oval, rather than circular, and each segment is three-
lobed instead of five-lobed, and the lobes themselves are not
approximate but divergent ; see PI. xc. fig. 7. The plant
which Ehrenherg figures as the young of his species seems
to be referrihle to the following genus.
63. H O LO C Y S T IS Hass.
Char. Fronds much compressed, formed o f two deeply divided
segments not incised. Segments not divergent. Lobes
angular, not radiant. Angles often spinous.
Derivation. From oKos, entire, and k v c t t l s , a cell; in allusion
to the fact of the segments being entire at their
extremities, and not emarginate as in Micrasterias.
This genus bears considerable resemblance to Micrasterias
Ag., from which it difiers mainly and essentially in the absence
of the radiant disposition of the lobes, which is the chief
character of that genus. To it should be referred perhaps
the Euastrum crux melitensis, pi. xii. fig. 3 c. of Ehrenherg ;
Arthrodesmus convergens Ehr. approaches likewise rather
closely to the genus Holocystis.
1. H o l o c y s t is o sc it a n s Hass.
Plate XC. Fig. 4.
Char. Frond large, smooth. Segments dilated below, contracted
above, and again dilated into a large triangular
lobe, which is somewhat rounded on its distal side. Angles
spinous.
Micrasterias? oscitans Ralfs, MSS. ; Jenner, in Flora of
Tunbridge Wells.
Hab. On Ashdown Forest, near Duddleswell : Mr.
Jenner.
Of this species I noticed several fine specimens mixed up
with other Desmideæ sent me by Mr. Sidebotham.
64. P E D IA S T R U M Meyen.
Char. Fronds compressed, circular, or star-like, each composed
o f several cells, arranged in one or more united concentric
circles. Marginal cells bipartite.
Micrasterias Ehrenherg.
Derivation. From t t zBío v , a bond ox fetter, and aarpov, a star.
The Desmideæ are a beautiful tribe of productions ; and
none of them more so than the subjects of the present genus,
which cannot be confounded with any other. Distinct, however,
as the genus Pediastrum is, considerable difficulty is experienced
in the determination of the species, owing to there
being no one character which can be depended upon absolutely
in their definition ; the size of the fronds, number
of cells in each, and the degree of incision of the marginal
cells varying considerably in each : there is, however, a certain
mean in these particulars which enables the observer to
determine in most cases the species or variety to which each ■
frond which presents itself to him really belongs.
Meneghini has noticed the occurrrence of transparent
vesicles in the cells of the frond, the number of these varying
according to the species : he has therefore availed himself of
these vesicles in their determination.
c c 2
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