il'
G
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y f
equally ventricose, arranged in a double, irregularly alternating
series.”
Arthrodesmus acutus Ehr., Infus, p. 150. tab. 10. fig. 19.
c, d ; Meneg. 1. c. p. 207. ; Kütz. 1. c. p. 609. fig. 96. ;
Ralfs, in Annals, vol. xv. p. 403. pl. xii. fig. 6.
“ I notice this species because the Rev. M. J . Berkeley
has gathered it near King’s Cliff, and I have occasionally
met with specimens which agree with Ehrenberg’s figures ;
but as I omitted to draw up a description at the time, I have
borrowed the specific character from Meneghini.”-— Ilalfs.
4. Scenedesmus obtusus Menegh.
Plate X C I I. Fig. 15.
Char. Cells three to eight, ovate or oblong, with rounded ends,
and arranged alternately in two rows.
Menegh. 1. c. p. 208. Scenedesmus quadralternus Kütz.,
1. c. p. 608. fig. 94. Scenedesmus octalternus Kütz.,
1. c. p. 609. fig. 95. Arthrodesmus acutus Bailey,
Amer. Bacil. fig. 18 ? ; Ralfs, in Annals, vol. xv. p. 404.
fig. 8.
Hab. In boggy pools, Dolgelly : Ralfs.
“ The specimens I have examined did not agree in every
respect with the description and figures of this species, hut
they probably belonged to it. The cells were ovate, the
broad ends of the rows placed alternately, the smaller ends
being in different directions ; sometimes the cells seemed only
held together by the hyaline matrix, in which state they
appear to connect the Desmideæ, through Gonidium Ehr.
and Trochiscia Kütz., with the Ulvaceæ. The cells of one
row are separated by the interposition of the broader ends of
the other.” -—Ralfs.
Note. Several of the figures of this family, especially certain of those
o f the genera Euastrum and Cosmarium, axe taken from those of Jenner
and Ralfs, illustrating the series of papers on the Desmideæ inserted in
the “ Annals.”
ALGÆ SILICAIVE.
F a m . XX. DIATOMACFÆ.
Char. Algæ silicious.
T h e Diatomaceoe form a group of natural productions not
less distinct and remarkable than the previous one of Desmideæ,
with which it may be contrasted.
The Desmideæ are Algæ of figured and peculiar forms,
binary composition, vivid green colour, destitute of silex in
their constitution, and therefore subject to considerable change
of form in drying.
The Diatomaceoe are productions possessing also, to a certain
extent, figured forms, but are not binary, of excessive
fragility, of a dark brown colour, which changes somewhat to
green on drying, of a metallic lustre and silicious composition,
and therefore unalterable in drying, and indestructible
by fire.
The silex which enters so abundantly into the composition
of these plants is disposed throughout the interior of the
• frustules in plates, which are often arranged in series, which
when the frustules are examined microscopically, are seen to
impart certain lines or markings to them, which contribute
much to their beauty, and which are of importance in the
discrimination of genera and species.
I t is thus seen how very little connection this group and
family of productions has with the undoubted Algæ which
have hitherto been described ; indeed, the claims which they
possess to be regarded as subjects of the vegetable kingdom
are by no means so conclusive as those which have been
advanced in favour of the vegetahility of the Desmideæ.