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DIATOMACEÆ.
verified niy observation on these minute striæ : when first I
observed them, I thought that they wmre found only in the
undulated variety of the plant, and that this therefore might
form a distinct species. I am now satisfied, however, tha t the
sti'iæ are to be found in every well-developed specimen ; and
the difference observed in the end view of the frustules in
certain species is indicative of merely a condition of the
plant.
A remarkable state of this species has been met with by
hlr. Ilalfs and Mr. Jenner. In this, within each frustule is
enclosed apparently a second frustule of a more or less oval
form, and striated laterally in the same way as the extremities
of the cells.
Occasionally it happens in this and in other species and
genei-a of Diatomaceoe tha t the cells in their front aspect
are not in the form of a parallelogram, but somewhat cuneate :
this occurs in consequence of the frustule being divided obliquely,
and not by a straight line.
The frustules, Mr. Ilalfs observes, have sometimes a central
pellucid spot, which does not appear to be connected wdth the
endochrome.
When recent, Fragilaria pectinalis is of a dark brown, hut
in drying it turns to a greyish green, with a metallic lustre.
2. F ragilaria hyemalis Lyngh.
Plate XCV. Fig. 5.
Char. Filaments very fragile. Frustules long. Sides slightly
emarginate, puncta very small. End view elliptico-lanceolate,
with from two to seven or eight well-marhed striæ,
which terminate in distinct puncta along the anterior terminal
margins o f the frustules.
Lyngb. t. 63. ; Ag. Syst. p. 7. ; Consp. Diatom, p. 63. ;
Kiitz. in Linnæa, 1833, p. 72. Fragilaria confervoides
Crev., in Br. Flor. vol. ii. p. 403. ? Harv. Br. Algæ,
p. 197. ; Kalfs, in Annals, vol. xii. pl. ii. f. 3.
Fragilaria turgida Ehr., Infus. pl. xv. f. 13.
FR A G IL A R IA . 4L
Hah. In freshwater pools and rivulets, Sussex : Air. Borrer.
Llyn Prefeddyr, near Barmouth : Rev. T. Salwey.
Aberdeen ; Dr. Dickie. Stevenston : Rev. D. Landshorough.
Ballantrae, Ayrshire : Mr. W. Thompson. Dolgelly,
Tavistock, and Trenteshoe, Devonshire ; Penzance,
Cornwall : ilfr. Ralfs.
This is a smaller species than Fragilaria pectinalis, differing
from it also in the slightly emarginate sides of the frustules,
the elliptico-lanceolate form of their end view, and the paucity
and decided character of the striæ seen on the end surface.
The filaments are so fragile, that the frustules separate on
the slightest touch, so that it is rare to meet with a filament
of any length. In most specimens the frustules are as long
as broad, and some even longer ; but in others, again, they are
two or three times broader than long. The filaments not so
unequal as in the other British species of the genus. When
recent. It is of a dark brownish colour, becoming whitish
brown in drying.
Thore can be no doubt of the correctness of the reference
to Ehrenherg.
3. F ragilaria rhabdosoma Fhr.
Plate XCV. Fig. 6.
Char. Filaments much compressed. Frustules usually many
times broader than long ; in the end view lanceolate, and
without striæ.
Ehr. Die Infus. p. 204. t. 15. f. 12. Frag, pectinalis
Kiitz., in Linnæa, 1833, p. 73. ? Frag, tenuis Ag., Conspect.
Diatom, p. 63. ? Ealfs, in 1. c. p. 108. pl. xi. fig. 4.
F. bipunctata, F. angusta, F. scalaris, and F. diopthalma
Ehr., t. XV. F. rhabdosoma Jenner, in Flora of Tunbridge
Wells, p. 202.
Hah. Common in pools and ditches.
The exceedingly narrow frustules, and the lanceolate form