Synedra Ulna Ehr., Infus. P- 211. pl. xvii. fig. 1. ;
E . Ulna Jenner, in Flora of Tunbridge Wells, p. 204.
Hah. Conunon in ponds and ditches throughout Britain.
An exceedingly distinct species, not likely to be confounded
with any other. The extremities of the frustules
would appear to niidcrgo the same change as has been observed
ill those of Diatoma elongatum, and to become
somcwliat dilated.
3. E xilaria fasciculata Kutz.
Plate X C V II. Fig. 3.
Char. Frustules small. In front view alternated at either
end ; in side view extremities acute.
Exilaria Vaucheria, E . fasciculata, in part, Kütz., in
Linnæa, 1833, p. 560, 561. t. xv. p. 38. 40. Sgnedra
fasciculata Ehr., Infus. p. 212. t. xvii. fig. 2. Exilaria
truncata Harv., in Manual, p. 204. Diatoma truncatum
Grev., in Hook. Br. Fl. p. 407.
Hah. Common in ditches, &c.
This is a very distinct little species : the frustules are often
in pairs, and owing to these being frequently b u t imperfectly
separated from each other, a curved or fan-shaped lamina is
produced.
4. E xilaria lunaris.
Plate X C V II. Fig. 4.
Char. Frustules curved, small, slender, converging at the
extremities.
Synedra lunaris Ehr., Die Infus. p. 212. pl. xvii. fig. 4.
Exilaria lunaria Jenner, in Flora of Tunbridge Wells,
p. 204.
A very distinct and pretty little species.
Char. Frustules straight in front view; curved in the fo rm o f
the letter S in lateral aspect; longitudinally striated, loith
a central depression.
Derivation. From gvpos, a curve, and cnyga, the letter S ;
in allusion to the form of the frustules.
1. Gyrosigma hippocampa Hass.
Plate CII. Fig. 11.
Char. Frustules in front view attenuated towards each extremity
; in side aspect lanceolate, curved, longitudinally
striated.
Nav. hippocampus Ehr., Die Infus. p. 180. t. xiii. f. 11.;
Frustulia hippocampus Jenner, in Fl. of Tunbridge
Wells, p 204.
Hah. Cheshunt; A. II. II. Olford and Shoreham, K e n t:
Mr. Jeriner.
In the same genus with this species should he placed the
Navicula sigma, N. scalprum, and N. curvula of Ehrenherg.*
82. N IT Z S C H IA Hass.
Char. Frustules curved ¿re front view in the fo rm o f the letter
S; in lateral aspect straight, transversely striated, and without
a longitudinal f u r r 0 1 0 and central depression.
Derivation. Named in memory of AI. Nitzsch.
The single species upon which this genus is founded
accords with Exilaria in all its characters, save in the seg-
moidal curvatures of the frustules.
1. N itzschia elongata Hass.
Plate C II. Fig. 12.
Char. Frustules linear, lengthened, with truncate extremities in
* The Sigmatella of Kiitzing, the construction of which term is somewhat
objectionable, is synonymous with Gyrosigma.
r F 2