ose ; annuii 5 to 9, 12 in -001" ; septa entire, alternate, increasing
in length as they recede from the valves ; valve, in smaller frustules,
elliptical, in larger, with a slight inflation in the centre, and produced
and linear ends ; striæ reaching the extremities of valve, 19
in '001". Breadth of filament 'OOOfi" to -0029". Breadth of valve
■0003" to -0005". v.v.
Kiitz. Bacill. xxi. ii. 4. Fragilaria striatula, Grev. B. F. p. 403. ad specim.
authen. in herb. Grev. Fragilaria Carmichaelii, tiarv. Man. p. 198.
Tessella Catena, Ralfs, Ann. vol. 12, p. ii. 1. ad specim. authen. in herb.
Jenn.
Marine. Seaford, Sussex, March 1852. Lancing, Sussex, Aug. and Nov.
1853, W. Sm. Swansea, Mr. Ralfs. Frith of Clyde, Feb. 1854, Mr. R.
Hennedy.
Plate XXXTIII. 306.
G e n u s 40. STR IAT EL LA, Ag.
Filament compressed, stipitate ; frastules imperfectly siliceous, annulate,
indefinite, annuli plane; valves elliptical-lanceolate, with a
median line, striated.
The stipes, which is longer in this genus than in any other of the
filamentous Diatomaceæ, is a feature which cannot be overiooked;
and the imperfectly siliceous nature of the frustules, the non-cellulate
structure of th e annuli, and the slight projection of the septa, which
increase a little in length as they recede from the valves, sufficiently
characterize the only species which has hitherto been detected.
The specific name is derived from the appearance of the endochrome,
which in the living specimen is invariably collected in a
central mass, with slender threads radiating in all directions towards
the cell-wall.
The coherence of the frastules is easily disturbed, and it is not
unusual to find some filaments in which an imperfect zigzag has been
formed ; in such cases a slight accumulation of mucus takes place at
the cohering angles. The filaments are ordinarily, however, direct
and continuous, and rarely consist of more than three or four, frequently
of only two frastules, in which respect Striatella differs from
Rhabdonema and the other allied genera, in which the filaments ordinarily
contain twenty frustules or more.
The figure given by Kützing, Bacill.pl. 18. fig. v. 3, representing a
smaller frustule attached to a larger, is manifestly an error arising from
the accidental juxtaposition of two frustules of different filaments.
The striæ on the surface of the valve are exceedingly delicate, and
require a higher power than the one we have usually employed to
bring them distinctly into view.
Striatella unipunctata, Ag. Annuli 12 to 36, 12 in -OOl"; septa
alternate. Width of filament -0022" to -0032". Breadth of valve
•0006" to -0009" ; striæ 68 in -OOl". v.v.
Ag. Consp. p. 61. Kütz. Bacill. xviii. 5. Ralfs, Ann. vol. 13. pi. xiv. 6.
Harv. Man.p. 109. Fragilaria unipunctata,'Ly'a^.TerA.\A\.G. Grev.
B. F. p. 405. Achnanthes unipunctata, Grev. S. C. F. tab. 287. Tes-
sella Catena, Ehr. Inf. xx. 7. Prit. Anim. iii. 180. Tessella pedicellata,
Dujar. Inf. xx. 14.
Marine. Torbay, July 1846 ; Lulworth Cove, Dorsetshire, Aug. 1847 ;
Jersey, Aug. 1852 ; Newhaven, Sussex, Sept. 1852 ; Larne and Belfast Bays,
July 1853 ; Poole and Weymouth Bays, Sept. 1854 ; Cork Harbour, July
and Oct. 1855, W. Sm. (Mediterranean Sea near Marseilles, May 1854,
W. Sm.)
Plate XXXIX. 307 : margin of a frustule, 307 x.
G e n u s 41. TE TRA C Y C LU S , Ralfs.
Filament compressed, with a central inflation, free ; frustules annulate,
indefinite ; annuli plane ; septa alternate, equal ; valves
inflated at the centre, cruciform, costate.
This genus, established by Mr. Ralfs in 1843, is easily distinguished
from most of its allies. Rhabdonema minutum and Tabellaria flocculosa
are the only forms with which the species it includes might be
confounded. I t resembles R. minutum in the F. V. of its filament,
but differs from that species in its cruciform and costate valve and
equal septa, and is easily distinguished from Tabellaria flocculosa by
its continuous filament.
The costæ in the present genus are usually pervious, and curved ;
occasionally, however, they are direct or interrupted.
The genus Biblarium, constituted by Ehrenberg in 1845, appears
to differ from the present merely in the solitary character of its
frustules, and this character arises from the fossil nature of the