Diatoma fenestratum Lyngb., Hydr. Dan. t. 61. E. 3. ;
Ag. Consp. Diatom, p. 53. ; Hook. Br. FI. vol. vii.
p. 406. ; Harv. Br. Alg. p. 202. ; Ralfs, in Annals,
vol. xi. p. 453. pl. ix. fig. 4. ; Jenner, in Flora of Tunbridge
Wells, p. 200.
Hah. Pools, &c. ; Sussex, &c. : Mr. Jenner. Cheshunt :
A . H. H. Barmouth, North Wales: Rev. T. Salioey.
Dolgelly, and near Pont-AberghasJyn, North Wales,
and Penzance : Air. Ralfs. Stevenston, Ayrshire : Rev.
D. Landshorough. Aberdeen ; Dr. Dickie.
This is one of the least variable of the species of this and
the following genus ; it may however be confounded with
Tabellaria flocculosa in the last stage of its developement ;
but with a little care it may be readily distinguished therefrom.
In T. fenestrata the frustules are connected by a perceptible
hinge, and on each fully developed frustule there are
at each extremity two striæ. In T. floccidosa tbe mucous
hinge is not visible connecting the frustules, which are usually
much longer, and on the narrowest, which approach nearest to
T. fenestrata, there is frequently but a single stria at one
side, and two at the other. The end views of the frustules
in the two also difier. In T. flocculosa the frustule is more
slender, the ends less incrassated, and the canal more prolonged.
The frustules in this species and in D. elongatum are often
curiously thrown back so as to form two lengthened series.
70. DIATOMA Ag.
Char. Frustules not having a central canal or lateral striæ,
punctated at the sides, the puncta being produced by the
presence o f grooves, which pass round the frustules. Junction
surfaces often striated.
Derivation. From Biarogy, incision; the plant looking like
a band cut into portions, which cohere only at the
angles.
1. D iatoma v ulgare Bory.
Plate XCIV. Figs. 1, 2.
Char. Frustules two or three times broader than long, sometimes
attached hy a stipes. Lateral puncta very evident.
End view inflated, incrassated at the terminations, and
striated.
Diatoma vulgare Kiitzing, in Linnæa, 1833, p. 582. f. 66.
D. tenue Grev., Crypt. Fl. t. 354. ; Berk. Brit. Alg.
t. 6. ? Hook. Br. Fl. vii. p. 406. ; Harv. Manual of Br.
Algæ, p. 202. D. tenue a moniliforme (Young) Kütz.,
in Linnæa, 1833, p. 580. f. 60. ? D. tenue /3 intermedium
Kütz., 1. c. p. 580. f. 61. ? D. flocculosum Ag.,
Consp. Crit. Diatom, p. 53. excl. syn. Dillw. Bacillaria
vulgaris Ehr., Die Infus. p. 197. pl. 15. f. 2.
Diatoma vulgare Ralfs, in Annals, vol. xi. p. 450. pl. viii.
fig. 8.; Jenner, in FL of Tunbridge Wells, p. 202.
Hab. Pools, streams, &c. King’s Cliff, Northamptonshire
: Rev. M. J. Berkeley. Henfield, Sussex : Mr.
Borrer. Shoreham, Kent, and several stations about
Lewes and Tunbridge Wells: Air. Jenner. Shrewsbury :
Mr. Leighton. Oswestry, Shropshire : Rev. T. Salwey.
Cheshunt, Herts : A. H. H. Ireland : Mr. D. Moore.
This species, like most of the Diatomaceoe, varies very considerably
in the length and breadth of the frustule, as well as
in the degree of its convexity. Usually each frustule is about
two and a half times as broad as long ; in this state the frustules
are seen to be very convex in the end view, but sometimes
older specimens are met with, in which the breadth of
the frustules exceeds some five or six times their length, and
in this they are scarcely at all convex when viewed endways.
This condition of the species might almost mislead one to regard
it as a distinct species. See the figures.
From Diatoma tenue, or rather D. elongatum, of which
D . tenue Is but a state, with which, however, there is but
little danger of confounding it, D. vulgare may be distinguished
by the greater breadth of the frustules, their con-
D D 4