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species of the genus Cymhella : N. amphora Ehr., N. lineolata
Ehr., and N. Westermanni.
1. Cymbella zebra Hass.
P late C. Fig. 8.
Char. Frustules in side view semi-lanceolate, truncate; in
front aspect elongated, quadrangular, with straight sides.
Striæ distant, strongly marked, double.
Navinda zebra, Abhandl. der Akademie d. Wissensch. zu
Berlin, p. 262. ; Bericht, der Akademie d. Wissensch.
zn Berlin, 1837, p. 53. Eunotia zebra Ehr., Die
Infus. p. 191. pl. xiv. fig. 7. pl. xxi. f. 19.
Hab. Cheshunt : A. H. H. On the moist conglomerate
sandstone cones near Cushendall, co. Antrim : Air.
Aloore.
This fine species approaches rather closely to the followino-,
from Avhich it difiei’s however in two respects; in the striæ,
which in Eunotia zebra are less numerous than in E . turgida,
and in the form of the frustule, which in the front view is
not attenuated at the extremities as it is in that species.
2. Cymbella turgida Hass.
Plate C. Fig. 7.
Char. Frustules semi-lanceolate, truncate at either extremity
in side view ; in lateral aspect narrower towards either
end than in the centre.
Navinda turgida, Abhandl. der Akademie d. Wissensch.
zu Berhn, 1830. Eunotia turgida, Bericht, der Akademie
d. Wissensch. zu Berlin, 1837, p. 45. ; Die Infus.
p. 190. t. xiv. fig. 5.
Hab. Storrington Common ; Air. Jenner.
This species needs only to be contrasted with Cymbella
zebra, from which it differs in the two particulars indicated
m the description of that species, viz. in its finer and closer
striæ, and in the frustules being attenuated iu the front view
towards either extremity.
3. Cymbella ? arcus Hass.
Plate C. Fio-. 6.
Char. Frustules smooth. In front view contracted towards
either end; in lateral aspect curved, attenuated, with a
projection in the centre o fth e concave margin.
Navinda arcus, in Wiegmann’s Archiv. fur Naturg. 1836,
p. 243, 2 4 4 .; Die Infus. p. 182. t. xxi. fig. 10.
Through this species the genera Cymhella and Navicula
would appear to be united, it agreeing with the first in the
semilunar and curved form of the frustule in its lateral view,
and with the latter in its attenuation in its anterior aspect.
C. Form o f frustules various.
79. N A V IC U L A Bory.
Char. Frustules straight. In front view usually somewhat
attenuated towards either end, extremities generally truncate
; in lateral aspect boat-shaped, extremities often
acute.
Derivation. From navicula, a little boat.
This genus, as limited to those species, which the above
definition is intended to include, would appear to be an exceedingly
natural one.
a. Frustules without transverse striæ.
1. N avícula phoenicenteron Ehr.
Plate C IL FIs'. 9.
Char. Frustules elongated, smooth, large. In front view
attenuated towards either end ; in lateral aspect extremities
lanceolate.