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w
i,; -îl;®
In the end view the frustules have been ajitly compared to
the quatrefoil o f a gothic wbidoiu : in the smaller filaments in
tliis asjiect, they arc about as long as broad, in the larger one
and a half times as broad as long ; and each has about seven
well-marked striæ, a central straight stria which extends from
one marginal lobe to the other, and three curved striæ belonging
lo each of the front and hack lobes.
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69. T A B E L L A R IA Shut.
Char. Erustules qnadrangidar, fusiform, cohering hy their
angles, traversed in the centre hy a longitudinal canal,
marked loith transverse * striæ, interrupted in the centre bg
the canal. End view destitute o f markings.
Derivation. From tabella, a le tte r; which the frustules resemble
in form.
The term Bacillaiia is applied hy Ehrenherg ^o the species
of this and the following genus, as well as to a curious
production wdiolly difieront from either genus, the Bacillaria
paradoxa, and to which species I propose tha t the genus
Bacillai'ia should now he confined. By Agardli, the appellation
of Diatoma is given to them. Mr. Shuttleworth
has proposed the division of the genus Diatoma into two
genera, in the propriety of which I fully concur ; for one of
these Mr. Shuttleworth reserves the name of Diatoma, that
of the other is derived from a species named by Ehrenherg
Bacillaria tabellaris, and which species is typical of the genus,
1. T a b e l l a r ía f l o c c u l o sa .
Plate X C IV . Figs. 9, 10.; P late X C V I. Fig. I I .
Char. Frustules in goung spscimens as long as broad, in older
specimens their breadth exceeds their length by tivo or three
times. Striæ from one to seven on each side. In end
* The word transverse has here a meaning different to that generally
assigned to it ; it here has reference to the width of the cell or filament,
and not to its depth.
view frustules dilated in the centre by the canal, with
rounded extremities.
Diatoma flocculosum Ag., Syst. p. 4. ; Kiitz. in Linnæa,
183,3, p. 5 8 4 .1 .17. f. 67. ; Hook. Br. Fl. vol. vii. p. 406. ;
Ilarv. Br. Alg. p. 202. Conf. flocculosa Dillw., Conf.
t. 28. ; Eng. Bot. t. 1761. Bacillaria tahellaris Ehr.,
Infus. p. 199. pi. XV. f. 7. Diatoma flocculosum Biúía,
in Annals, vol. ix. pi. ix. fig. 3. Bacillaria flocculosa
Ehr., Infus, t. xv. f. 9. B. seriata Ehr., Infus. t. xv.
fig. 8.
Ilab. Common in pools, &c.
This is a very variable species, and it is not to be w’ondered
at that a difference of opinion should have existed as to
whether the conditions of it represented in “ Eng. Bot.” and
in Dillwyn’s “ Confervæ ” were not in reality distinct species.
That they are not so, however, is certain, as well as tha t both
are different stages of the growth of the same production.
In the young state, the cells are as long and even longer than
broad, each side of the frustule being marked with often as
many as seven well-marked striæ ; and the central canal is
large and circular. From this state intermediate specimens
are often met with, some having the frustules once and a half,
others twice, and others thrice, and even many times as broad
as long. Corresponding with this gradual lateral enlargement
of tlie frustules, we find the number of the striæ to diminish
gradually, until at last but one or two exist on each frustule,
and at the same time, a gradual diminution in the size of the
central canal occurs, until at length this is nearly obliterated.
Thus the ultimate stage of the species bears some resemblance
to Tabellaria fenestrata, and is the Bacillaria seriata of
Ehrenherg. F o r these changes see the figures.
2 . T a b e l l a r ía f e n e s t r a t a .
Plate X CV I. Fig. 10.
Char. Frustules fo u r or fiv e times longer than hroad; in end
view subinflated in the centre ivith incrassated ends.
I) D 3